Ok, I'm a guy but I know how it feels since I've done it with my gf. It can be a pleasurable experience if you know what you're doing.

1. Perform enema on her. It can be messy, but it shows her that you care. She'll love you more and won't back out.
2. Buy KY jelly or water based lub.
3. Lots of foreplay or intercourse to make her real wet.
4. Tell her to RELAX. She has to be calm in order for her anus muscle to open up.
5. Lube up your hand, her hole. Use a lot.
6. Use one hand to touch her clit. The other hand, use the index finger goes SLOWLY into her hole. Back out if she says it hurts.
7. After feeling the hole loose up. Add your middle finger. Continue push and pull them out.
8. Slowly easy your penis in. VERY SLOW.
9. Go slow for 10 strokes. Add more lube while doing it.
10. Start your engine and watch her moan.

Every girl is different so don't expect your girl to like it. I like anal sex because it's so tight. I feel like making love to a virgin again.
In my opinion, the best way to do it is doggie style. You need to move forward a little bit so your penis will strike her G-spot from the anus. Tell your girl to touch her clit. That'll send her over the edge very fast.

Expect your girl to have trouble sitting after and trouble pooping for a day. She'll be on the toilet wanting to poop but nothing comes out.

That's my experience. If you think it's dirty. You're wrong, there are preparation in it. You don't go straight to it, then it's dirty.

Sorry it's in English but it takes forever to type in Viet.