This is one method I've used that has worked with a few ladies that have graced me with the pleasures of intimacy. I am "not" claiming to be a sexologist, doctor, or such. This is not meant to be "absolutely" correct method, but it has worked for me in the past.
I would say first you should do all the things you know she likes. Make sure you've got her good, hot, and horny. (NOTE: She most likely would want to have had some "really hot" intercourse before you ever attempt to enter her anally.)
After intercourse, eat her out some more til she's smokin' hot with lust, while she's laying on her side or on all fours. Next, wet her anus with her own juices that you gathered up with your hand or use your tongue. Make soft, small wet circles around her anus with your finger or tongue. If she moans with pleasure and doesn't say, "Get the f#@k out of there right now!!!", you may be on your way to a very pleasurable experience, provided you don't rush her. (Warning: You may "not" get it that night!) Just let her give you the signals. Ask her if she likes what you are doing? Think about "her" pleasure. Try to get her even more aroused, if so far she's liking it.
Regardless, if she wants to try something else, then give her what she wants. She may be more open to anally explore further the next time you have sex with her. Shows her that you respect her limits and are not just after your own sexual gratification.
Okay. After the finger/tongue session, let's say she still feels like exploring anal sensations that evening with you. Maybe it's time for some real lubricant after that. Gently work a finger up there with plenty of lube. Does she like it? Yes? Good. Take your time (at least 15 more minutes or more. She's a virgin back there possibly.)
Above all, try to keep her clit stimulated throughout your session. If all is still good with her, try to get your lubed-up thumb up her anus very slowly and rhythmically after you have gently removed your finger from the area. Maybe she'd prefer a very small, well-lubed smooth-shaped toy up her anus. Above all, keep stimulation enjoyable, especially on her clitoris. Maybe initiate intercourse with her while you have a very small toy in her anus or your thumb. Again, make sure she is well lubed! Make sure she is still finding the experience pleasurable.
Next, make sure your erection is "extremely" hard. Position yourselves into the spooning position or have her lay flat on her tummy. Remove your thumb or toy from her anus gently. Add more lube to her anus! Gently, but firmly press the head of your cock against the entrance of her anus without entering. Just press the head of your cock against her anus firmly enough for her to know you are there. Make sure you aim right!
Now, wait a couple of minutes while still applying a small amount of pressure. Next, make small in/out motions against her anus "without" penetrating her anus. Watch her body language. Listen to her moans. If she's enjoying this motion, then firmly, in a single motion fully push the head of you cock into her anus.
Now wait! Keep a firm pressure against her anus with your cock head inside of her without moving in deeper yet. Let her get used to the sensation for a few minutes. When she appears to be somewhat accepting of it, make a few slow insertions of only about an inch deep. Again, watch her body language and listen to her moans. After her anus starts to relax, make your cock jabs a little faster going into her "no deeper" than an inch. Keep a steady pace. Her anus should open up a little more from this motion.
Next, when her anus opens up a little more yet again, push your cock up into her another whole inch further. And again, slow down your jabs till you feel her ass muscles open up further. After she relaxes her ass a bit more, you can jab a little faster, making sure you are only going two inches deep in/out of her anus. Once she's relaxed enough at this interval, it's time put it in considerably deeper. Maybe about half of your cock's length in one slow firm steady movement. Stop and wait for her to relax before going back into motion.
Once inside to that point, go slow again till she is good with it like that. Afterwards, you can go at a medium steady pace with your cock half way home. When her ass opens up a little more again, you can push it home! Then stop again, while keeping your cock firmly and fully planted inside her for a minute or more depending on the lady's preference. Then go slow, deep, and all the way at a steady pace.
Once she is rocking rhythmically with pleasure to the sensation of your cock, go a bit faster. Watch her body language and listen to her moans to gauge from there if you should still go faster and/or harder.