In general girls prefer a steady rhythm. They want you to speed up the more excited they get. When you move your fingers very fast and then slow down they are back a few steps.
When you start out you can tease the girl, not stroking the most intimate parts right away, touching the clitoris and inner lips only lightly, changing the positions of your hands and the technique. But once she is very excited you should touch her the same way as long as possible, making the same motions and keeping a steady rhythm. This is the only way for most girls to have an orgasm. If this is too tiring you can use a vibrator.
Most girls want you to go on stroking when their orgasm starts. If you stop too early it can be a bummer. Some like you to keep moving your fingers the same way, some want a little more or less pressure and there are also girls who don’t like the movements but only want the pressure. Girls differ in this respect, so you should ask her.