How would you touch a girl's booty?

If you're not already dating, then the answer is: at your own peril.

More seriously, if she's not your girlfriend, then be prepared to apologize for even the most casual touch, as that level of unwanted intimacy is considered a violation of personal space; and outright groping could be considered harassment or even assault.

If you are dating, the rules are a bit different. You touch; if she objects, you stop; if she doesn't, you touch some more (until she says she's had enough).