Bathtub masturbation is great, as said above. Let the water run over the whole area down there and it feels great.
If she just wants to use her hands, tell her to spread the lips down there and just start playing with herself- literally playing with herself. She should be stress-free (I guess thats what the people above are getting at with the candles and all that). Find the clit, and put a little pressure on it, and then start rubbing it in little circles. I typically hold open the labia with my left hand, and the third and fourth finger of my right hand. Then I rub with my index finger. If it's a little dry, have her lick her finger and then start rubbing again. But that's just me.
But really, I would advise getting a vibrator. They are so sexy during sex and feel really good during masturbation. Have you ever tried oral sex on her? The feeling of a tongue flicking down there feels great.
Hope I helped a little.
