Oral sex is an art not a science. It's a fun thing to do and the more you do it the more you'll be able to read a woman and know when you increase what you're doing and when to slow it down.
Do not treat it as a science. Do not look at too many diagrams. It does not work that way.
My favorite thing about giving a woman oral sex is the potential for teasing. It turns a woman on so much but obviously it's not really turning you on. So unlike with sex, you can just stop. She'll be sexually frustrated, but you will not be bothered. I thoroughly recommend stopping, when you can tell she's really turned on and just walking out the room to annoy her. Then come back and finish the job. (Not something to do on your first time though – wait until you've got a little experience under your belt.)
Rub the entrance to her vagina with your fingers but do not penetrate it. Done right this will tease the hell out of her.
Lick her clitoris and write the alphabet with your tongue. The constant changing of direction will turn her on a lot.
Remember the clitoris is very sensitive, never bite it.
You can however put your lips over her clitoris and 'munch' it with your lips. This feels great to a woman. Also try blowing on her clitoris. For some women this will do nothing, but for the more sensitive ones it will feel amazing.