To teach bow command, put your dog in stand position, hold a treat on your left hand and slide your right hand under his belly like above to block him lying down or moving forward. (You may ask someone to hold the treat for you if your hand not long enough) move the treat along the floor level to lure him move further with hint leg stationary. Repeat the command when teaching.
After his head almost reach floor level or in correct bow position, repeat command 'bow' and praise after few seconds.