Well I am not a chick, but here is a great way. Spend a lot of time eating her out, as you are doing so work you way to her ass, she wont really even notice. Lick her ass as much as possible, stick your tounge all up in there. Show her how good it feels, she will freak. Then make sure she is really wet, finger her pussy, and get lots of juice all over her, and swish it down to her asshole. Go back and forth licking her ass, and getting it real wet. Then slowley, and I MEAN SLOW, stick you tip of your finger in while you are licking her clit etc. This will drive her crazy. Start from there and work your way up. It takes time, but she needs to know how good it feels!