Here is a text guide for those that don't like to watch videos. They are in map order. Please let me know if I need to explain any of them more in the comments, and don't leave bad feedback without leaving an explanation, so I can fix it.
As an FYI: You do not need to complete the track once you get the squirrel. Once the number x/20 pops up in the top right, you can quit out and move on to the next.
1) D License - Wait until a squirrel falls on you after you finish the track.
2) Sunset Stripped - At the start, reverse into the fence. A firework will go off. Go through the track without faulting until you get to a closed fence. Bust it open, then land in the crate.
3) Strike - After going through the first building, fall down the crack between the first and second buildings. Land on your wheels, and go through the open door, up the ramps, and to the rodent.
4) Cliff Jumper - Drive slow off of the first set of rocks after the first jump, and fall on the section below and go backwards until you have it.
5) Rock Steady - Run into the barrels at the end of the track, and they will explode, and eventually the rodent will show.
6) Sewage Plant - Once you pass the 3 shooting sewer pipes, back up and go off of the next ramp leaning forward, and bail at the end of the ramp and you will go above to where they are.
7) Suspense - When you get to the big swing, stay on it for an extra swing, and it will swing higher to a hole in a higher platform. Land up there.
8) Titans Graveyard - After going through the Cage, the next big jump fall onto the rocks below, and go backwards until he pops up.
9) Meltdown - Reverse at the start, catch on fire, and complete the track without faulting.
10) Archipelago - Just do 5 flips without faulting. As soon as you do your 5th flip on the run, the rodent appears on the back of your bike (Even if you crash while doing the 5th)
11) Mind Bender - Go through the cube of water and before you exit it, bail out and you will go up and back to the left to the rodent.
12) Harbor Havok - After the first big climb, the second container you land on will be open. Reverse into it.
13) Power Plant - Once you go up the elevator, reverse until you land in the water.
14) Swamp Fever - After the second hanging mine, around the 6th checkpoint, go up the big hill before going into the building and bail out before the top and go up and over the building.
15) Physics Factory - Once you get on the spring pad, reverse until you fall in the box.
16) Around the Ranch - At the end of the track, reverse before the two water troughs right before the finish line, reverse.
17) Redlynx Moto-Jam - Drive toward the first set of logs until fireworks go off, then you have to reverse until you get the rodent.
18) Lab Rat - On the part with the pool of water(6th checkpoint). Press up, up, down, down, left, right, left, right, A, X, A, X.
19) Under Construction - When you get to the part with the metal railings and things falling around you, once you get to the upper railing, reverse back down the track without crashing to the pit that is right after the 2 rectanglur metal beams (After the big block of concrete)
20) Bridge of Trials - On the next to last checkpoint, jump high on the ramp and bail to a zip line. Hold Y to hold onto it. (Note: If you hit the last checkpoint then you have to restart the track)