Another thing you can try is get a small-medium-large toy and slowly increase in size during a session. This is what I did with my girl before anal sex and after 3 months I can slip in at any point with a little lube.
Everyone has posted good advise at this point. I only have one thing to add. Increase diameter only a quarter inch at a time. Don't ever try to make huge increases in girth, especially if you're near or over 2 inches in diameter.
lube! and try leaving things in at first with each sized toy, then move to thrusting when you're comfortable, it feels so great and it opens you up but you know the toy isn't too big because you used it before! If you have a partner, gentle finger play is helpful, like if your partner puts two fingers in and stretches them apart a little...
I would recommend getting an anal beginners kit. It goes from small, medium, large, to a full on dildo usually. I know I started slowly from small and went to medium and i'm working on getting to a large. Use LOTS of lube. You can never use too much lube when it concerns anal play. Definitely get a good material you are comfortable with to try anal with so you have less pain. Do not push yourself too hard. Go slowly and relax. You'll do fine. xoxo
Toys are great for this. I suggest starting small, maybe with a 1" diameter toy. As others have said, move up 1/4" at a time. And yeah, lots of lube, lots of patience. With practice, it's possible to work up to just about any size.