When I taught my dogs how to bow I did it the way I wanted LOL. That's actually the way I teach my dogs all their silly tricks and they do fine. Basically, I do whatever I can to get them in the position I want and hold them there while I go nuts saying "GOOD *Insert trick here*!!! YAY!!"

For bow I put my dogs in the stand position. I put one hand under their belly. The other hand is holding a treat and I say "bow" and direct them downward towards the floor. My dogs got it really fast.