Lower the seat to the point where he/she can comfortable put his/her feet on the ground. My sister started by sitting on the seat with one foot on a pedal, and the other "scooting" her along (kinda like a skate boarding motion). She went up and down the road propelling herself along in that manner until she got a feel for the balance, and gradually started picking up that foot and trying to rest it on the pedal for a second or two until she felt more confident. Sorry, I'm not explaining it better. It took about 30 minutes for her to actually be riding "properly" down the street (i.e. pedaling, not scooting). She felt a little dorky (with little kids zipping around her and all), but she got comfortable very quickly with the balancing by scooting herself along.

here's another link:
http://forums.teamestrogen.com/showt...dult+ride+bike

Hope that helps,
Christy