There are 16 squirrels spread across 16 different levels. Collect each one to unlock this achievement. The levels listed below go in order from first to last. Lastly you will need multiple playthroughs to grab all the squirrels as there are often squirrels in both decision towns when you get to them on your map.
Here's how the squirrel guide works: Level: Description of how to get to it.
Cabot Ridge: In the same room as Jess Collins in the cabin. The squirrel is sitting to the left of the fireplace, once your cursor goes near the area the squirrel will glow white.
Sedalia: After exiting the store, go up the ladder in the alley. Go to the right side of the roof to cross to the other building. Inside the door and down the ladder.
Pemberton: Heading towards the motel, go into the gas station on the right, the squirrel is on top of a shelf in the middle of the store. Enter through the back.
Garwater: Inside the fire station. Enter through the door on the far left. On a desk inside the first room.
Fontana: Right near the start there will be a fenced off area in front. Jump on top of boxes on the right to get over the fence. After you are over this fence there will be 2 walkers and then a pickup which you must jump ontop of. Once you're up there you will see the squirrel next to a dead man in a chair near the edge.
Cleburne: In the morgue, which has about 10 zombies feeding on dead bodies when you walk in, it's in the back left corner on top of a hospital bed.
Oakview: After you push the car to block the fence, follow the path to the right of the tents all the way down. Inside the tent at the end of the path, the squirrel is in the back right corner.
Taggart: Behind Charlene's house, in the shed.
Lemon Hill: Go down the alley where Merle tells you to look for fireworks. Up the ladder at the end, walk forward and fall down in the gap between buildings. The squirrel is on the left end.
Barksdale: Enter the donut shop from the street. Go through the hole in the wall in the back. Jump over the counter, it's behind the counter.
Lafferty: In the grocery store, past the locked room with the poster there is a square area with cash registers around it in the center of the room. The squirrel is on a cart in the center of it.
Archer Creek: After you are told to get into the building to the left of the blue car in order to get cover from the snipers go to the second building on your left. If you are following the poster guide as well, from the poster turn right and go between the cubicles. The last cubicle on the left has the squirrel.
Danvers: Once you enter the large room full of cubicles, go down the left path. The squirrel is at the end of this path next to the left corner cubicle.
Polksville: After you get inside the green train car, it's inside on the right.
Sherwood: The second house on the left from the start. The one with Harrison and his wife in the bedroom. It's in a corner in the garage.
Firesign Stadium: After turning the corner past the burning tank, you will be in a open area with a large tent to the right and a fence with cars in front of it to the left. Go all the way down following the left side and you will see a lone tent. Go through the fenced off area to enter the tent. The squirrel is on a table.