You don't have to be crazy to find every squirrel in Big Sands, but it helps!
There are a total of 10 squirrels in the Riders of Doom DLC. They are available at any time, using any bike although I recommend the Phoenix. Just make sure to collect all 10 of them to unlock this achievement, and you can quit out of the map as soon as you unlock it.
Special thanks to "K4rn4ge" for compiling this list!
1. - [0:06] - Greek Therapy - Easy - Choose any bike available to you. For this squirrel you can't fault. At the very start, you will see a small ramp with a fountain to the right of it. Ride on the fountain and you will hear a "ding" noise. Keep going until you get to the third checkpoint and prepare for a big drop. You will want to land on the 2nd fountain and make sure you hear the "ding" again. Keep going until the 7th checkpoint and you will see the third and final fountain after the next small ramp. Ride on this fountain and you will now hear a different melodic sound. This is the signal that you have opened up the secret chamber. Keep going through the level still with 0 faults until the big gap before the finish line. Instead of jumping across it, slowly fall down into the newly opened chamber in the floor. You will go backwards down the ramp into SQUIRREL #1
2. - [0:53] - Toxication - Easy - Choose any bike available to you. Get to the 8th checkpoint. You will hear a creaking sound. Once you hear that signal, fall down the incline backwards (the way you came up before). You may need to backflip in order to get back down but you will need enough speed to also go over the pile of barrels to your left. Once over those, keep backing up and slide down another incline into the toxic waste. Wait a few seconds and you will see SQUIRREL #2 pop out of a small pipe which wasn't there before.
3. - [1:25] - Anno Domini - Medium - Choose any bike available to you. Get to the 2nd checkpoint. There will be an incline. Instead of jumping across the big gap fall down inbetween the two columns. You will notice the left column starting to collapse. At this time you will have to gain speed on the small ramp and do a backflip off of the column on the right. You will have to time it so that you go over the falling column and then land on the left side. Wait there and the camera zooms in a bit and you will see a squirrel pop out of the cup/mug for SQUIRREL #3.
4. - [1:48] - Halloween Hot Rod - Medium - Choose any bike available to you. Get to the 4th checkpoint. Underneath you, you'll see a pumpkin hanging from the ceiling. You must have that pumpkin fall on your head and from that point on you CANNOT fault. You are able to fault at that checkpoint until you get the pumpkin on your head but NOT after that. To get down there is a bit tricky as you will have to hook your tire onto the plank and swing around to land under the pumpkin. From that point on you cannot fault and must navigate through an alternate version of the track. Towards the end you will go up an incline into a previously locked room revealing SQUIRREL # 4.
5. - [2:59] - Trouble at t' Mill - Hard - Choose any bike available to you. Get to the 2nd checkpoint. You must perform two flips at any time. You will hear a bird chirp each time you flip. I recommend doing the first flip either at the big drop directly after the 2nd checkpoint or at the 3rd checkpoint over the stairset. For the 2nd flip, you will probably want to flip and land in the big gap with the wheel and water. Once you have done your 2nd flip, the water will drain and you will fall into a capsule. SQUIRREL # 5 will then pop out of a crate.
6. - [3:23] - Black Site - Hard - Choose any bike available to you. Get to the 9th checkpoint. You will need to input a code with the following controller button combo. It is: LB, RB, Click Left Stick, Click Right Stick, Up direction on Right Stick. As you input the code, you will hear beeps and see colors light up the board in the back. If you entered the code correctly, you will hear a spaceship sound. Now you must make it through the rest of the track without faulting (there are only 2-3 more obstacles before the end). If you fault, simply reload the checkpoint and enter code again. After going past the finish line, just wait. A spaceship will beam you up and you will get SQUIRREL # 6.
7. - [4:02] - Dystopia - Hard - Choose any bike available to you. Get to the 13th checkpoint. Back up a bit and you will be on a small platform that sticks out from the wall. Get a bit of backwards momentum and fall back down to the left. Try to land on the flat platform if you can. Once down there you will see a checkpoint from before. Let the backwards momentum kick in and you will go down another incline. Once there, you will see a squirrel hit a switch to trigger an explosion and you will get SQUIRREL # 7 and if you got ALL 10/10 squirrels, you will be rewarded with the "The Squirrels Have Nuts" achievement!
8. - [4:46] - Overpass Action - Hard - Choose any bike available to you. Get to the 7th checkpoint. On the right you will see a fence with a down arrow sign. Do NOT go down. Instead, you will want to press Y to bailout and go over the fence and light pole and land on the highway. A red car will start speeding towards you. You need to try and land in front of the car before it runs you over. You can keep trying from that checkpoint as you have to be fast but yet you still need to bail high enough to go over the light pole. If you land in front of the car early enough, the car will stop and a squirrel will peek his head out of the car window for SQUIRREL # 8.
9. - [5:07] - Bazaar Run - Hard - Choose any bike available to you. Get to the final checkpoint right before the gap before the finish line. Instead of simply riding off the ramp into the finish line, you will want to get enough speed to go up the ramp and then at the height of your jump, press Y to bailout. Try and aim your rider to the right and over the broken wall with a barrel that's above the finish line. Up there lies SQUIRREL # 9.
10. - [5:30] - Nature Calls - Hard - Choose any bike available to you. Get to the 9th checkpoint. You will notice a small wooden platform/raft in the water below. You will need to land here and wait on it for about 10 seconds. The raft will start to move and you will reach an area with SQUIRREL # 10. NOTE: When the raft starts to move, you will automatically fault but obviously this is intended as your body will float on the raft to the squirrel location.