1- Communicate and Get them relaxed
"Firstly, remember that if you’re playing with another person, you need to feel comfortable communicating so you’re both having the most enjoyable experience possible," . Make sure that you’re listening to and responding to your partner’s body. If you’re not sure they’re happy, ask them."
The sphincter muscles of the anus are very strong, and need to be given time to relax for comfortable anal. "You also need to be in the right headspace for that to happen," . "If the brain’s not willing, the body won’t be either." Spend time to eat his ass ,indulge in long foreplay before penetration
2- Use Enough lube
You need need NEED to use a good quality lubricant designed specifically for anal. The Anus needs extra stuff to make penetration comfortable, more enjoyable, and to protect you and your partner from injury. ''Saliva is not lube''
3- Locate his P-spot
Most guys just slide in and thrust immediately .Why not locate his spots first? The key to anal orgasms is prostate stimulation. "You can feel the prostate gland a few inches inside the anus on the wall closest to the stomach as a firm, round, textured area about the size of a walnut, it generally requires firm (but not rough!) stimulation for maximum pleasure."
Try fingering before your go in. You may notice that during prostate play, there’s a lot more pre-cum that comes out before ejaculation occurs - this is a result of what is known as prostate milking.
Try to add a bit of passion during sex, try playing with his nipples , his neck region and the tip of his penis as you thrust in and out .