CRUSH letters appear throughout all the Hill Climb events: Time Attack, Stunt, and Survival.
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.
Collecting all five letters in a Time Attack level will be easy. You might need to manage your speed a little, slowing down or reversing to get hard-to-reach letters. Give a level a few tries and you'll learn their locations.
Pause and choose to restart if you miss a letter while going for this Achievement. Or don't worry about it, because you're bound to get all five at some point as you progress through 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.