What do you do when you put your hair in a bum and the little hairs in the back are too short? Do you cut them off or leave them?

I assume you mean "bun" and not "bum". I there are short hairs in the back that don't fit into the bun I just use a bobby pin to pin them down. It's that easy.

I use Bobby pins and some hairspray

use bobby pins and/or hairspray. always does the trick. I recommend both tho

You use inconspicuous barrettes, clips, or bobby pins to hold them in place.

Bobby pin them up. :)

i clip them up or leave them

i boby pin them