When we first tired it, we used Vasaline as a lubricant, and I started with my fingers. My little one, then worked up to 3 fingers over the course of play. We did this several times before she felt ready to try it. It is a muscle, it will stretch. We also used a vibrator to help stretch, slow entry. plenty of lube, again working my fingers in before the vibrator, it was a small one. Once she got to be able to take the vibrator, then one night after the vibrator we tried me. I was on bottom, she on top and could control entry. Another thing was an enema, she to to like those too. It has been a long time ago since this occured, but I do remember how she slid down and at first was just pushing herself against me, and rubbing, then she pushed a little harder and got the head in, then she laid there and adjusted to it, then she started moving her bottom with me in her. Finally, she said, Ok, you can move now, and I did, still taking it easy until she was ok with me being there. It got to be her favorite way of having sex. I still remember the first time she asked for that as opposed to regular sex, I had licked her for about 30 minutes, was getting ready for a normal entry, and she said No, I want it in my butt please. I did, but I was surprised she liked it enough to ask, and that was when it became available for regular use. Sometimes we would put the vibe in her vag and me in her other hole. she really liked that, or have two vibes in her and she giving me a bj.
Just go slow the first few times, and have him on bottom if you want to control entry speed and duration. Use fingers the first few times until you learn how to relax that muscle easily. Hopefully you will find it as pleasant as she did.
Forgot to say we also used mineral oil, baby oil, think we used hand lotion a couple of times, cooking oils would also be good. Not sure I would use KY, it is water based and has a tendency to dry out.
try it firs just rubbing, you don't have to penetrate the first time, get used to the feeling, or feel the feeling, just play with it a bit before you decide to do the penetration. To me that was the real excitement, learning the new feelings, and yes, she did it to me later.