See the article Anal Sex – Basics for the fundamentals. This article will cover some details of anal sex, in particular in contrast to vaginal sex.
The first major distinction is the stop and go pattern of regularly applying lube and then putting it in and thrusting, especially at the start. Refer to the original article, but it’s important to have a lot of lube there, the anus does not lubricate itself, and this is aided by taking some time to periodically switch from thrusting and applying lube on your dick.
Remember that the anus is further back than the vagina. This makes the position of her lying on her back often the best logistically, and also allows her to relax and be still. Thrusts should start and remain shallow for a bit of time before going further in. The ass takes much longer to stretch than the pussy, so refrain from sliding in completely in the beginning, typically.
A similar position that also works well is in a “side cuddle” where she is your little spoon, and you thrust in from there. Typically strokes should be even, steady, and straight. The simple stimulation comforts her mentally in being predictable.
From behind is also a solid standard position, and remember that smacking her ass relaxes her anus so you can play with this, like thrust in far a few times then.
When she is on her back, her legs must be lifted farther back than otherwise, and pelvis rotated up. I do this for vaginal sex too, but in the case of anal it is critical to have a pillow under her hips lifting them up at the appropriate angle.
Don’t forget to regularly apply lube, even as you start to get into it. And on this note, your mileage may vary. Some women can eventually work up to receiving vigorous intense pounding type of penetration. This can take some tiime, however, and typically, even getting the whole dick in completely is uncommon at first. Ultimately, slowly build up the intensity and watch for her to wince when you should ease back a bit, then maintain and slowly build up againi.
Once it opens up, the asshole is more rigid and stays a bit agape in between thrusts, so ironically it is tighter at first but gets looser for the serious anal warriors. In this case, a fun thrusting technique is to start changing the angles, working around clockwise or side to side, etc. in terms of the angle from whiich you slide into the asshole. Also, maintain the mindset that you are always digging further, so pound a bit harder then grind your dic farther and farther in, continuing to expand it if ever so slightly.