First rub your thumb around on her asshole.
Then slide it in.
If she's still alright with it, stick your whole thumb in and thumb-fuck her ass.
If she let's you thumbfuck her ass. There's about a 50/50 chance she'll let you stick your dick in there.
Ass fucking isn't really that great though. It's just cool once in a while to so something different and freaky.
If the woman has been eating a lot of food, ass fucking can be REALLY nasty. Real life isn't like porn movies where they have had an enema and the inside of their ass is totally clean and empty of shit.
Fudge packer approved