Here is in my experience what works well. I am a licensed sex educator (LSEd) and a professional cunnilinguist
(joking)
1)

2) slow and gentle up and down motions along the vulva to begin for a few minutes. You can slowly build up the rhythm.
3) start to focus on the clitoris. Up and down and/or in circles around. (Google image search vaginal anatomy to help locate the clitoris if necessary)
4) keep focusing on the clitoris and stay there all the time. No need to go wild with your tongue, but keep going!
5) If the stars align she may get an orgasm. This may take anywhere from 2 to 15 minutes. Don't be shy to ask her what she prefers in terms of technique and for instructions along the way!