Make sure her clit area is well lubed, then touch the vibrator there with it on its lowest setting. Ask her how it feels, if she wants it stronger or if it's too intense and she wants a layer of fabric between or something. If it has different kinds of throb settings you probably want a plain low-power buzz at first. Throbs are better for pushing someone over the edge to orgasm, IMO. You can move the tip in circles around her clit or hold it still. The most sensitive spot is probably just below the clit, matching the way a man's cock tends to be most sensitive on the underside, where the head meets the shaft.