how do i teach my cat to clean her butt?

How old is your cat? She may just not have the hang of it yet or when you got her she may not have been taught yet by her mother. What you can do is monitor her bathroom use and spray her bottom with a bit of water because this will cause her to want to clean the area. You can also just wipe her butt yourself.

Lol, my Tabitha used to do the exact same thing, but stopped after I shooed her off the bed. First time it happened, my warm blanket smelled like pee and had a tiny poo stain. I got up, yanked off the soiled blanket and shoved it in the machine. Ph--eew! The cat did it a few more times before she finally got the message that she was to stay off the bed until she cleaned herself --- thoroughly.