Practice, practice, practice. Ask the girl you're with what works. Ask her what doesn't. Bear in mind that what works with one person won't always work with another (some women have very sensitive labia and like them to be licked, others have much less feeling there; some women need more stimulation on the clit, some less.) Don't forget to use your lips (and even, if the woman you're with likes it that way, your teeth) as well as your tongue. Your hands are also important parts of the whole. The more you do it the more you will learn.