I'd say the first thing to do is find out a way to enjoy vaginal. If you're anxious going into anal with him because you know vaginal is uncomfortable you're setting yourself up for failure. Don't move into anything new that you're worried about until you find a way to enjoy what you're doing already. Try different positions, relaxation exercises, whatever.

Other than the basic recommendations for working up to buttplay with a large toy or penis (look at some of the other threads for these), the curve means that anal sex will be more comfortable if you match the curve of your rectum to the curve of his penis. Look at an anatomy book or website. While I'd like to be able to point you to one of my blog posts that would answer your questions, I don't have one . The closest I can get is here link, with a few pictures, but you might be better off checking out link or link (while the last site is targeted towards gay men there is good information for anyone on this page, though if you're trying to get a reluctant spouse who thinks anal play is "gay" to let you play with his ass it's probably not the best place to start).