so many answers already but I wanna add my 2 cents!

I love anal and honestly the best way to do it is to prepare before you do it.

get some water-based lube and work it around the area and onto your fingers and then slowly inside, you'll know when you are relaxed

grab a plug (a small one maybe) then lube that up too and slowly work like 1/4 inch in and out for a while.

Take as long as you need to relax! When you feel comfortable with the 1/4 inch then work in another 1/4

Do that until you can work it in to the widest part of the plug and don't go past it! Just work in and out to there since once you get past the wide part it really just 'pops in' cuz your ass wants to close and it's so slippery from lube.

Make sure you hold on to the end basically and just work it until you are relaxed and comfortable.

Then you can grab a bigger plug if you feel the need and repeat the previous.

Wear the plug for a while (10 minutes, 20 minutes, an hour, whatever you need) then when you have sex, you're ass will be open and tell him to go slow

have fun!