This solution is based on a guide written by Monoxide Child in 2006, with the locations of COG Tag 28-30 added by Creech on 12 May 2008. I have added the missing or incorrect chapter names to make it easier to follow.
You can pick up COG tags in any order, on any difficulty and in a Co-Op Game.
When you have collected 11 COG tags you will get
When you have collected 22 COG tags you will get
When you have collected all 33 COG tags you will get Act 1: Ashes14 Years After E-Day
COG Tag 1 - Right after you make your choice of paths, there is a tag straight ahead under the Gear logo. Can't miss it.
COG Tag 2 - In the room where the locusts cut through the door. If you are looking directly at the door they cut through, you can find it back and to the left. It's under the stairway across the room you came down.
COG Tag 3 - As you exit the above mentioned door into the courtyard, there is a stairway to the left. Run up the stairs and turn left right at the top. The COG tag is in the near side corner (if enemies are North, it would be in the SW corner of the patio area.Trial By Fire
COG Tag 4 - your team points this one out to you. In the first small courtyard, look to the right of the Gear logo.
COG Tag 5 - You cross a bridge over a road and enter a building. Once you enter the building with all the pillars and the two emergence holes, it's just behind the second emergence hole in some weeds.Fish In A Barrel
COG Tag 6 - You fight through the roadways witht he Trioka guns and get to the open area with the fountain in the center. THere are 4 emergence holes here. The COG tag is in the far left corner (your left as you enter the area)Knock Knock
COG Tag 7 - You're told to gain entrance to the house of sovereigns. You'll enter an open area with stairs leading up to the building. To your far left you'll see a white van with the Gear logo painted on it. Follow the massive hole in the street on the left all the way up, sticking to the edge. The COG tag is in the corner made by where the wall extends over part of the stairs.Hammer
COG Tag 8 - In the hallway where Jack has to open the door, there are a ton of annyoing wretches. If you're facing the door that Jack is opening, it's in the far left corner. Alternatively, you can follow the wretch footsteps on the floor right as you enter this hallway and they'll lead your right to it, just to the left of the door you came in through.
COG Tag 9 - there's a small room that you enter where you'll find the body of Rojas. As you come down the stairs into this small area, turn right immediately. THe COG tag is behind the pillar to your right.China Shop
COG Tag 10 - Beginning of the China Shop chapter - The Berserker kills a soldier. Go back into the area where he was killed, and you can pick up his dog tag near his body. It's in a dead end alley on the left, near some flames.
COG Tag 11 - There will be 3 smashable doors. as you proceed through this area. You may feel a bit of stress as you're doing this. To the left of the third smashable door, in a little alcove, immediately before you exit the Tomb area.
COG Tag 12 - Just after COG Tag 11, you exit into an open courtyard. Follow the semi-circle wall and head left. The tag is on the other side of a block just after you see the logo. If the room was a clock, the Tag would be between 8 and 9, right against the semi circular wall.Act 2: NightfallTick TIck Boom
COG Tag 13 - After your choose a path (either one, doesn't matter) and reunite, You'll enter a small room with two stairways in it, one in the very middle of the room that leads down to a couch. Shoot up the couch, and it's underneath.Grist
COG Tag 14 - There's a room with a catwalk that square shaped. WHen you exit, you head down some stairs into the street. Turn around and run back the alley formed by the stairs you came down and the wall of the next building. The Tag is at the end of that alleyway, behidn a rock.
COG Tag 15 - Just before you enter the Settlement, you come up the stairs where the Boomer was and its behind a newspaper machine. Facing ahead as you come up the stairs, it'll be to the back and left (the exit area will be to your right).Lethal Dusk
COG Tag 16 - On your way to checkpoint 2, facing the first emergence hole, you'll see a propane tank in a shed to your left. Shoot the tank and enter the shed.
COG Tag 17 - When you enter the area where you have to use the spotlight, after you guide Dom through, move the spotlight to the far left (as you're holding the light) area near where you entered. The COG Tag is in that corner area. just to the left of the lit room of the blown out buidling, on the street.Dark Labyrinth
COG Tag 18 - After you leave the destroyed building, look directly to the right. It will be on the ground in the light.Act 3: Belly Of The BeastDownpour
COG Tag 19 - In the open, rainy area next to the sea, there is a dock behind the buidling with the elevator in it, on the far right. Get down on this dock and follow it all the way out. Tag is at the end.Evolution
COG Tag 20 - In the room with the rotting floorboards, the Tag is in the far right corner (as you enter the room).
COG Tag 21 - When you enter the room with the carts, go down the stairs to the car area and turn around. The Tag is to the left of the stairs as you face them.Darkest Before Dawn
COG Tag 22 - After you ride down the drilling platforms, stick to the wall on the right. There will be a path leading down to the right, follow that to find the COG tags and some grenades.
COG Tag 23 - In the next area after Tag 22. you'll enter an open area with a small building on the right side. Just to the right of the stairs leading up to that buildling, you'll find the tags.
COG Tag 24 - After you choose your paths, and reunite, you'll fight through a narrow hallways with a lot of cover. Just after the hallway and before you exit the area, turn around and the tags are on a narrow ledge over the pool. (to your right as you leave the hallways, or to your left after you turn around)ACT 4: The Long Road HomeCampus Grinder
COG Tag 25 - As you enter the first area, on the right there are two small staircases. The Tags are behind the second small staircase.Bad To Worse
COG Tag 26 - As you enter the second area, before you go down the sidewalk with the columns to enter the conservatory, there is a rusty car on in the street. If you're facing the columns, it's back and to your right behind that car.Imaginary Place
COG Tag 27 - In the second section of the estate, there's a shield shaped room (square top, semi circle bottom) with 4 columns in it, and a desk. Destroy the desk. Tags are underneath.ACT 5: DesperationComedy Of Errors
COG Tag 28 - Quite a ways into the PC-only levels the Brumak will be stomping above you as you go through a tunnel filled with wretches. When you come out, you will be in a garden and directly in front of you is a half open gate. Behind that gate to the left is a cog tag.Window Shopping
COG Tag 29 - At the end of the parking garage you go up some stairs. Before you do that, go down stairs for the cog tags.Jurassic Proportions
COG Tag 30 - When fighting the Brumak, go all the way to the back of the fight area and look for the cog tags near some ammo and a washing machine.Train Wreck
COG Tag 31 - It's on the left side of the 5th train car, after the last door is opened.
COG Tag 32 - in the third passenger car on the right, before you go up the ladder.
COG Tag 33 - There's a room on the left side of the fifth and final car in the third part of the train. It's at the end of that room.