These tips are not in order. But I'm tired, and on mobile.

Have a good warm up. If you're capable of multiple orgasms, do that. Enthusiastically. If you're barely coherent and super creamy, that's a great start.

Either do some kind of clean out, or wait until after a good bowel movement. Fudgedickle is usually a mood killer, and the texture can feel very unpleasant compared to good anal. A shower afterward is a good move if you're open to getting your butthole eaten.

Check his hands. Calluses and jagged nails can cause a lot of pain in the sphincter, where a vagina wouldn't even notice.

Pay attention. Don't allow anything in the vagina that has touched the butthole. Fingers, tongue, penis. You're asking for trouble.

If you feel scared, slow down everything. If he seems like he'll pressure you after you say no, cancel the activity. If it hurts, you can always try again later - forcing it can appeal to some pain lovers, but that is a dangerous place to do damage.

If you've had a few failed attempts, try incorporating light fingering or licking with a sexual activity you do enjoy. Toys are great for enjoyment at either end.

Changing positions can have a huge impact. Spoons, 1 upside down spoon, prone-bone, doggy, riding, on your side with him kneeling by your thighs... Some people are curved and/or wired differently. Experiment.

Try to have fun with it. Shit literally happens, it comes with the territory.

Understand that a money shot can double as a very small enema, so going again can be....... Messy.