I suggest that she make herself comfortable with pillows under and around her, and you make yourself comfortable with a pillow underneath your hips, or with her on the edge of a bed or chair and you on the floor. Ask her to tell you when you are pleasuring her, and begin exploring her vulva with your mouth. Try lots of different strokes with your tongue and lips, and try not to feel as though you have to bring her to orgasm. If you find a spot that she particularly likes, stay there for a while. Don't worry about bringing her to orgasm, and don't be surprised when you do.
Oral sex can be a slow, pleasurable feast.
The idea is to use the silkiness of the tongue and mouth to produce wonderful sensations. Ask her to tell you when to stay in one place, and how fast she wants you to lick. All, both, and it depends. Some women think they take too long to have an orgasm, and may prefer to use a vibrator to get them there. Others love a vibrator in their vagina or anus, or both, plus oral stimulation to the labia and clitoris.