It depends on the circumstances. I may get into the shower or I may hang my butt over the tub or sink.
The basic method is to keep washing it and rinsing it off and then washing off my hand, until I no longer feel poop or anything resembling it on my butt. Be gentle. No soap on the butt, that will dry it out. (As I mentioned in other posts, I don't use soap anywhere besides my hands.)
I then keep washing it, occasionally washing off my hand. When I can rub my butt with my hand it comes back not smelling like poop at all, I consider my butt to be clean.
If you look at animals, they do something similar, though they have to lick it clean. I consider myself very fortunate and blessed to not have to do that. :)
Afterwards, I wash my hands, get dressed, and then wash my hands again.
Here is a challenge for you, get some dog crap on your hand and spray it with a garden house then sniff your hand. Unless you are soaping and scrubbing your ass deep, it isn't going to be 100% sanitary regardless of using paper or a bidet. They are nice, but in the end it doesn't matter.
I haven't used an actual bidet in a while, but I honestly find it hard to fathom not washing my butt after pooping now that I've started doing it.
TP is just smearing it around...