CRUSH letters appear throughout all the Hill Climb events: Time Attack, Stunt, and Survival.
Hill Climb has four maps: Forest, Desert, Polar, and Harbor. Each map offers seven levels per game mode, for a total of 84 levels.
Every Hill Climb level has five letters (C, R, U, S, and H) spread throughout the level. You must try to collect the letters and then complete the level. Medal rating doesn't matter, but you'll only keep the letters if you cross the finish gate at the end of the level. Die or restart and you'll have to pick them up again.
There are a total of 480 letters spread across the maps and game modes. Play through all of Time Attack, as that's necessary to unlock the levels for the other modes. You'll probably want to play the levels that unlock specific vehicles (several of which have Achievements).
Get all the letters you can, but don't get hung up on getting them all in every single level. Just keep going through levels in the various modes until you hit 100 letters. You'll probably hit 100 letters way before finishing Time Attack.
Progress towards the letters Achievements can't be tracked through the Achievements app, unfortunately. But it shouldn't take terribly long to get them all.