Take your time, and make sure you are both feeling turned on before you venture south. If you aren't used to giving head, don't expect to bring her to a climax in seconds flat. Ask what she likes You don't necessarily have to go down on her for several hours, - just a few minutes of oral sex can do wonders for your foreplay. Ask her what feels good and if there's anything that would make it better. Make sure you are both in a comfortable position, or you'll end up with an unsexy crick in your neck.
Every female is different.
There is no point in rigidly following those 'Ten Easy Steps to Give Her an Oral Orgasm' articles in lads' mags. It isn't like an exam, with right and wrong answers. Sometimes you have to experiment to see what works for both of you. Start gently Many women say that oral sex gives them more stimulation than penetration. So remember to take it easy and avoid being rough, especially to begin with.