Most women masturbate by rubbing a finger or two over their clitoris, sometimes through the skin of their inner or outer lips, in a circular or back-and-forth motion. You can do this too, and it is most helpful to ask, or better yet, have her show you how she likes it done. Lie down side by side, and put your hand over hers while she shows you what she likes. You can also try having her hand over top of yours guiding.