The way to actually get this achievement is rather self-explanatory, but there is a strategy that can make it easier. Place every letter block in a corner, remove one from the corner, and solely bounce that one with the metal crate. Since the achievement only requires one letter to be bounced, only using one letter makes it much easier to focus on bouncing that one letter. Moving the other letters to a corner minimizes the chance of you accidentally bouncing another letter at the same time, potentially breaking your focus on the original letter (especially if you're bouncing the two E's).