Positions!

My boyfriend wants me to try reverse cowgirl and honestly he is the second guy that I have had sex with so I am very uncomfortable in my abilities when it comes to my sex skills, I'm just afraid of doing it wrong! I have been dating him over a year and he loves our sex. Can someone explain to me how to do reverse cowgirl?! I feel so dumb asking I just don't want to do it wrong and ruin the mood!