Hm. You might be a bit more explicit. It's not all that different from any other kind of oral pleasure. There's the target zone, the anus, and the areas all around it - the perineum, the buttocks, the crack of the ass. That little dimple at the base of the spine. So just like any kind of oral pleasure, you can go straight to your target, the anus, and just lick around there, or you can sneak up on it, lick and kiss the surrounding area, move in on your target. Once you get there, you can just lick the anus or you can put as much of your tongue inside as you can fit.

Inexplicably (oh, not really) some people find this a bit squicky. You can do it while you're sharing a shower, or after a bath, or you can start with a nice warm washcloth. It feels good, and unless he's super sadistic, he won't mind a bit.