Anal sex! One of the most subtle pleasures that a woman can get (men too but that I am not expert at).
A French author once said that " if it wouldn't be made for penetration it wouldn't be round." Quite a shortcut but an interesting way to clarify.
I started enjoying anal sex when I was a student. It was with a woman and a toy. Which in a way is a very nice and delicate way to experience it first as a woman will stay soft and in control. Many men once they made it in your ass lose control :-)
You want her to move from "neve" to "take my ass"?
It will require steps, a lot, before you get to empty yourself in her ass. Yes I write in a crude language, I noticed it helps men to better understand.
Have you picked her tiny hole for more than two seconds? Have you slowly pushed your tongue 👅 in it? Start soft.
You should also start rubbing her anus gently with a finger from time to time when you are in her pussy.
Let her slowly discover that pleasure.
You can also get her some toys like a rosebud ...a small one. Let her get used to it in her own.
She has to become the one wanting it in her.
Be patient ... Let her be the one asking you to fuck her ass. But do it very very nicely the first time as it will hurt her a bit.
By the way ... Try yourself with some toys! Good for you to learn what she will experience.