Update Feb. 1: All videos and text descriptions are in the solution now.
There are three skulls in every level, for a total of 75 skulls. Most can be obtained on the first playthrough of the level, but others require you to come in through an alternate entrance. The difficulty in getting to each skull varies drastically, with some explicitly requiring a gadget or second player and others simply being much more difficult without one. I'd recommend using the Peg gadget until you can unlock the Attractor gadget (found in level 3-3) and then using the Attractor exclusively.1-1 Woodland Vestibule
1. Entrance A: Fall down a hole where a sign indicates to do so.
2. Entrance A: At the bottom of a large hill (you can't miss it).
3. Entrance A: Visible up and to the right as you approach the exit.1-2 The Forked Path
1. Entrance A: At the first crossroads, take the left path and go up and around.
2. Entrance A: At the second crossroads, take the left path and go up and around.
3. Entrance B: When you get to the dark area with the lantern equipped, take the lower path. It's directly underneath the sepulcher.1-3 Mist Falls
1. Entrance A: After the first long death rail ramp, go straight up. There is a co-op challenge, and the skull is past that. You will need to anchor the chariot with a Peg or second player while it's resting on a death rail in order to stand on it and reach the platforms leading to the skull.
2. Entrance A: After the second long death rail ramp, go up and to the left. There will be a death rail above you. You will need to anchor the chariot with a Peg or second player in order to get under a wall blocking the next platform. Then continue ascending and follow the death rails to the skull.
3. Entrance A: This skull is directly above the exit. Proceed up the platforms on the left side, then cross a small gap to get up there.1-4 Sunlit Underpass
1. Entrance A: Go left at the first crossroads. There is a platforming segment leading to the skull.
2. Entrance A: Just after the segment where you carry the chariot across a life path and dip it underneath death rail walls, let it drop and go down. There's a short corridor where you must pull the chariot through a few life path walls to reach the skull.
3. Entrance A: Near the exit, in the room with tall rock columns, take the chariot across the floor instead of going over. A tunnel leads to the skull room where you must pull it over platforms. You will want to use a Peg or second player to anchor the chariot so you can reach the skull platform.1-5 Lantern Way
1. Entrance A: After you open the first horizontal gate and fall down, go to the right. A series of platforms lead up towards the skull. There is a co-op challenge sign nearby, but it doesn't apply to the skull.
2. Entrance A: As you fall down a hall near the end, there is an opening on the left to access this skull's room. You will need to anchor the chariot near a life path wall, while you go around it.
3. Entrance B: Entrance B is unlocked via the alternate exit in 3-4. You can't miss this skull coming from this entrance, it's directly above the switch to open the door.2-1 Forgotten Forest
1. Entrance A: At the first crossroads, turn right and go down a narrow tunnel. Continue upward when you get a chance and turn left at the top; the skull is a little farther that way.
2. Entrance A: If you go left instead at the first crossroads, you will eventually reach a room with an upward slope and diamond patterns of gems in the air. You'll need to ascend the platforms on the right side and pass a little puzzle. Simply let the chariot roll down the platform while you jump through the death rail wall and catch it from the life path on the other side. Continue left, then upwards to find the skull.
3. Entrance B: Entrance B is unlocked via the alternate exit in 2-5. Take the platforms up from the entrance room rather than rolling down the hill, and you'll find the skull at the top.2-2 Tenebrous Hollow
1. Entrance A: After the blueprint crate section, you'll ascend a vertical room. Look for life paths on the right, and bring the chariot down there to find the entrance to the skull room on the right.
2. Entrance B: Entrance B is unlocked via the alternate exit in 2-4. Go to the right of the opening room and find a switch. This opens the gate to the left underneath the entrance; follow that path to the skull.
3. Entrance B: Stay on the top path and proceed to the right. If you fall down a shaft, return to the previous checkpoint. At the upper right side of the map you will find the skull.2-3 Maze of Shadows
1. Entrance A: After the first switch puzzle, continue to the right and you will eventually see a small switch underneath a brittle wall. Bring the chariot up the platforms and drop it on this wall to break it, then go down and press the switch. This opens a gate further to the right where the skull is located.
2. Entrance B: You will roll down some death rails right near the entrance. After you go off a ramp facing left, jump up and the skull will be in an alcove.
3. Entrance C: Entrance C is unlocked via the alternate exit in 3-2. Stay on the top path and continue right and you will eventually reach the skull. When you see a co-op challenge sign and gold switches, you are almost there.2-4 The Hive
1. Entrance A: This skull is located above the center of the map. After you cross over the central pillar of the level on a death rail rollercoaster, double back and find a checkpoint in the middle. Jump up the series of platforms there to find the chamber where the skull is located.
2. Entrance A: When you get to the top on the right side, you'll roll down a huge death rail ramp. Dangle underneath the chariot until the rail levels out, then pull yourself up before the chariot goes off the jump at the end. You'll need to be going at high speed to reach the life path platform leading to the skull (unless you have the Attractor, which makes it much easier).
3. Entrance A. As you get closer to the exit, you'll see a short death rail that curves up a bit, just before a checkpoint. Push the chariot over this death rail and lower it down, then anchor it with a Peg or second player. Then dangle from it and swing to find the life path on the right. This lower path leads to the alternate exit. Stay on the platforms near the ceiling here and you will get to the skull.2-5 The Prismatic Eye
1. Entrance A: Near the end of the level you'll reach the bottom half of the center of the map. Press both the player switch and the chariot switch to open the path up into the chamber where the skull is located. Watch out for looters!
2. Entrance B: Entrance B is unlocked via the alternate exit in 3-3. Near the end of the level you'll reach the top half of the center of the map. Carry the chariot onto life paths and drop it onto the brittle wall covering the chamber where the skull is located. Watch out for looters!
3. Entrance C: Entrance C is unlocked via the alternate exit in 5-4. Simply follow the upper path, the skull is located in front of the gate to access the secret exit. It will display the number of skulls you have out of 72.3-1 Frost Peak
1. Entrance A - About halfway through the level you'll slide down a ramp and approach the center chasm on an ice ledge. Jump off the chariot and anchor to keep it from going over the edge - you'll need to get down and cross a series of platforms to the left. Do not attempt this without the Peg or Attractor!
2. Entrance B - After you cross the top of the level on a guided death rail, you'll slide down a long, long ramp to jump over the central part of the level. If the chariot goes off the jump at full speed and you jump off at the right time, you can land on a life path and pull the chariot up. Continue to the right from here to find the skull. This one may take a few tries, return to the previous checkpoint if you miss the landing.
3. Entrance C - After the first long section where you do platforming underneath the chariot on a guided death rail, you'll have to pull it up a hill. Take the chariot into a cave here instead and put it on another guided death rail, and it will eventually pick up the skull. You just need to follow along the convoluted path so that the chariot isn't lost before it can get the skull.3-2 Chasm Crossing
1. Entrance A - Simply go left from the entrance and you will find this skull in a small room. Continue left instead of going down.
2. Entrance A - Midway through the level you will see this skull in a room after crossing the chasm from right to left. Go down, avoid the looters, and pull the chariot up the wall to reach it.
3. Entrance B - Entrance B is unlocked via the alternate exit in 5-1. After you slide off a death rail and land on an ice slide near this entrance, you'll hit a curve that slides to the right. The skull is right under this section; check your map to make sure you don't miss it.3-3 Frozen Trench
1. Entrance A - After the first few rooms where you put the chariot on a track, you'll see a downward hill, with guided death rails above it going against you. You'll have to pull the chariot past these rails using the life path platforms above them. The skull is on the other side.
2. Entrance A - Midway through the level you'll need to pull the chariot from above, over a gap between two death rails. After you've done this and gotten the checkpoint after, double back and fall onto the life path underneath those rails. The skull is floating underneath the path on the right side.
3. Entrance A - Just after a checkpoint there is a guided death rail that goes up at an angle, then straight to the right. This rail launches you at the skull if you get enough speed on it.3-4 The Crystal Castle
1. Entrance A - At the first checkpoint, double back and take the platforms up (you may want to use the Attractor to make it easier). You will reach a guided death rail going left that passes through four brittle walls; this ramps the chariot onto an icy slope. The skull is at the top.
2. Entrance A - At the checkpoint next to a brittle floor, continue right. At the end of the next room you can see the skull behind a couple more brittle walls. Break them by dropping the chariot on them, then go inside to get the skull.
3. Entrance A - Near the end you'll find a room with brittle floors. Break the floor by dropping the chariot on it, and fall down to a checkpoint and guided death rail. Ride the rail up to a ledge at the right side of the room, then follow the platforms up and left.3-5 Speeder Slide
1. Entrance A - At the end of the second sliding section. You must be going a decent speed in order to break the brittle wall guarding the skull.
2. Entrance A - At the end of the third sliding section. You must be going a decent speed in order to break the brittle wall guarding the skull.
3. Entrance B - Right at the entrance there are some guided death rails. Right them around until they send the chariot up near the skull, then jump onto the life path to pull it the rest of the way.4-1 Magma Springs
1. Entrance A - At the top of the first big slope in the level, you'll see a two-star sign. Take this path, it leads to the skull. You will have to separate from the chariot for some life path/death rail puzzles, but no gadgets are required.
2. Entrance A - Midway through the level you'll see two slanted life path platforms. Right after this, drop down to a lower level (watch out for lava!) and go left. You can boost onto the platform here or tilt the chariot up against the wall and jump off it. Take this tunnel further left and you will eventually find a ramp up to a maze of looters. The skull is at the top.
3. Entrance A - Just before you drop down to the bottom section of the level, there are some slanted platforms to the left which you can ascend. It's tricky, but when you get up to the top there is a long slope to the left. Take it upwards and you will find a looter maze with the skull at the top.4-2 The Molten River
1. Entrance A - In the chambers with the bounce pads, the skull is in an alcove in the top left of the first chamber, use the bounce pads to get the chariot way up there.
2. Entrance A - When you get to the section where the lava raises and lowers, the skull is in the area periodically covered by lava. Easy to get if you don't mind being roasted.
3. Entrance A - In the large room at the midpoint of the level, take the platforms up and to the left. You can see the skull in an alcove in the left-side wall. If you have the Attractor you can boost up there; otherwise, take the death rail up and to the right to find the path leading to it.4-3 Ali-Baba's Cavern
1. Entrance A - After the section with the blueprint crate, you'll get to a crossroads. Go left to find a sealed gate. Take some platforms up and there will be a very strong bounce pad to the right; the chariot will pick up the skull if it bounces off one of these pads. (I recommend going along with it.)
2. Entrance A - When you get to the second chamber where you seal a gate so the lava rises, double back and go down. You'll go around and under to another room with lava that flows left strongly. Go up, then take the platforms to the right to reach the skull.
3. Entrance C - Entrance C is unlocked via the alternate exit in 5-2. This skull is visible on the map from this entrance; simply go down a bit and it will be on the left.4-4 The Geyser of Fire
1. Entrance A - In the first main corridor where the lava is rising, continue up until you reach a fork. The skull is up in an alcove to the right.
2. Entrance A - Just after the first checkpoint, the skull is in the lava on the right side of the corridor. Ignore the platforms leading up and go right, dangle the chariot in the lava to get the skull.
3. Entrance B - Entrance B is unlocked via the alternate exit in 4-5. Directly under the exit for this part of the level is a ramp leading to the skull room. Just take the chariot over the platforms above the lava. At the end, push the chariot on its end against the wall, then jump off it to reach the platform the skull is on.4-5 Volcano Cove
1. Entrance A - When you get to the top of the volcano, fall inside. The lava raises and lowers once every 60 seconds or so. When the lava is all the way at the bottom, take a tunnel to the left on a speedy death rail, then ascend the platforms before the lava rises again. Ride the lava up to the skull.
2. Entrance A - Midway down the volcano there is an opening on the left. You'll need to break a brittle wall by dragging the chariot into it, then use the chariot to push a switch. This opens a gate on the right side of the volcano at the lava's high tide point. Follow the cavern that way to reach the skull. (This path leads to the alternate exit.)
3. Entrance A - Just after the volcano, there is a series of platforms leading upwards. Follow these, and at the crossroads at the top of the level, go left. Eventually you will reach a death rail that slopes away from you; you will need to anchor the chariot with a Peg or second player, then jump onto it to reach the next platform. Follow another, longer death rail down to break a brittle wall leading to a couple of looter rooms. The skull is in the top left corner of the second room.5-1 Eldertown
1. Entrance A - After you roll down a ramp away from some looters 1/3 of the way through the level, look for a pair of ice corridors blocked by a life path and death rail. Turn left here and follow the platforms. You'll get a checkpoint, then a timed-switch puzzle. Let the chariot roll onto the switch from the left and pull it up through the life path, then rush down and around to the gate. Follow the tunnel down and you'll find the skull.
2. Entrance A - Shortly farther along you'll see a brittle floor just before a checkpoint. Break it by dropping the chariot on it, and follow the tunnel down to a corridor with the skull at the end.
3. Entrance A - There is a bit about 2/3 through the level where you use bounce pads to get up to a death rail. Go left from here and you will eventually see the skull behind a gate (triangle symbol). The small switch is located in a room underneath it.5-2 The Throne of Sand
1. Entrance A - Early in the level you'll go left through a long room across some balance platforms. When you get to the left wall, you are supposed to drop the chariot down to break a brittle floor underneath, but instead bring it up into a corridor above the room. There is a skull a little to the right.
2. Entrance B - Some ways after you open the locked door, you'll come to a room with no hazards and a checkpoint on a narrow platform. There is a switch on a platform to the left of the checkpoint; it opens a gate in the lower right corner of the room. Go through a small tunnel there to reach the skull.
3. Entrance B - Right before the sepulcher, you ride the chariot off four speedy death rail ramps. When you hit the wall on the left side, double back down and boost over a sand pit with the Attractor. You'll see two gates surrounding a switch. Go left to find a small switch to open the left gate, and the switch opens the right gate. The skull is just beyond.5-3 Crumbling Pillars
1. Entrance A - At the top of the first long ramp, you can continue going up using balance platforms and life paths. When you get all the way up, go left and then around to follow the platforms leading to the skull.
2. Entrance A - At the blueprint crate, go up the series of platforms, then to the right into the room above the long ramp. There's a lightly sloped death rail; use your weight to pull the chariot up it. When you get to the end, use the Attractor to boost yourself up through the death rail and onto the life path, from where you can pull the chariot up to get the skull.
3. Entrance A - At the second long ramp (which you roll the blueprint crate down), stop at the end and fall down to the left instead of going off the jump. There's a guided death rail leading straight to the skull.5-4 End of the Line
1. Entrance A - A short way into the level you'll go down a long ramp and be launched over a long sand pit. Near the end of the chariot's arc you can jump off (or boost with the Attractor) and land in a cave in the right wall. Pull the chariot up to get the skull.
2. Entrance A - At the top of the level, just after the room where you ascend balance platforms with flying looters chasing you, the skull is above the checkpoint. Go left and take some more balance platforms up and around to reach it.
3. Entrance A - This skull is in the same room as the alternate exit. Midway through the level you'll get to some platforms in a wide open space, one with a checkpoint. Jump up onto the platform shaped like Florida; you'll see a signpost indicating up and left, but ride the chariot down and right instead. You'll be deposited onto a death rail which launches you towards the room in question. The balance platforms in here are very fast, so avoid letting the chariot touch them as much as possible. Use the Attractor to get up onto the platform with the skull.5-5 The Dreambreaker
1. Entrance A - At the top of the room with the ice platforms, there are some small platforms leading off to the right. Take these to get to the skull room, a diamond-shaped chamber full of round ice platforms. Cautiously use the Attractor to boost yourself to the top, where you'll find the skull.
2. Entrance A - When you get to the second vertical chamber, you can drop down to get access to this skull room. NOTE: There is no way back from here, so try to boost over the checkpoint at the bottom using the Attractor. If you get this checkpoint, you'll have to quit and restart the level once you have the skull.
NOTE ADDENDUM: LewisNoobBuster points out:
"If you look to the far left there is a spring, but it is only a decoy (you can't go high enough). But look to the background- there is an arrow pointing in the quick sand with a question mark; this is a mega spring that will shoot you right up top
First, let your chariot sink where the arrow is pointing, and then just as it has been submerged, jump onto it. If you time it right you will be shot up with the chariot. Took me a good few tries to get it."
Anyway, after the checkpoint down here you'll find some downward-sloping lava. Take the life path platforms to the left and drop the chariot into a narrow gap under the wall on the left. Jump onto the chariot and boost left with the Attractor, making sure not to fall in. The skull is just a little further to the left.
3. Entrance A - In the final huge chamber with the looters, you'll get to a checkpoint near a signpost pointing up and right. Go left instead, and follow the very tricky platforming bits, and you'll eventually get to the third skull room. This room has single-space rock platforms and wedge-shaped ice platforms with sideways-facing bounce pads. Avoid these ice platforms, the bounce pads make it impossible to do any platforming. Use the Attractor to bounce yourself up between the tiny platforms, and you can get to the skull at the top of the room.
*Note that you need the other 72 skulls before you can access level 5-5. Refer to the solutions for