How to do oral?

I have been dating my boyfriend for a couple months now. We are in a long distance relationship so when we do get to see each other, sometimes just for 24 hrs once a month, things get spicy. I'm a virgin but he's not. Last weekend he fingered me for the first time and it felt good. Next weekend he's coming again and he's spending more days to see me dance at my show. Earlier today, I made a joke about us taking a nap, shower and eat a doughnut together. He then replied by saying that he will eat me and I'll eat the doughnut. I really think that it will happen next time I see him. I'm excited and I want to experience that with him but I'm very self conscious of my body. I couldn't even reach an orgasam when he fingered me. And usually I'm able to orgasam when I masturbate. I feel that by doing oral sex I'll feel so insecure that I won't be able to orgasam. Any tips? Also what should I do before and after to prepare my vagina?