First, you should make sure that she is in to the idea of using Ben Wa balls. Some people do have a fear that things, such as Ben Wa balls, might get stuck inside of them. Even if they don't openly admit to the fear, a subconscious fear could tighten the pelvic muscles enough that it will make it hard to remove the balls.
Once you and your girlfriend discuss it and if she is OK with it, then you pick out a set of Ben Wa balls. Many have retrieval strings which should help relieve any fears about them becoming stuck inside. The short answer on how to use them is that you simply put them in. She uses her pelvic muscles to keep them inside. If, for whatever reason, you all do have a hard time removing the Ben Wa balls, you can try the following proven methods to get them out: