First time doing oral.. Advice?

Hey all, I just got into my first real relationship with a guy about a year ago, and we've started getting more and more intinate. It's p gradual but he's gone down on me twice now and I feel bad because I haven't returned the favor. I've never done it before and I'm really nervous about it. I want it to be perfect for him and I'm really, really scared to mess up. Advice?