Gosh, isn't a demonstration sufficient? And then warn them that the cuticle can get hurt. Tell them not to push too hard or keep things too stiff. Be relaxed, glide along the surfaces of the keys, and smile. When the glissando is finished, return immediately to a straight face.
I can teach an octave glissando, you know. I did it once, and the girl was amazed that now she had an octave glissando. Obviously you bend your thumb so the nail hits the next key down. The rest has to do with the position of the arm and distribution of weight. And while you're getting started, licking your fifth finger helps you play your first glissando or two. You know, lubrication.