When teaching the forehand yield, first get into position. Your hips should be angled as if you are about to walk under the horses neck and around to their tail. Next, settle your hand closest to the horse's head onto the knot of the halter and gently extend your hand forward while stepping in towards their neck. If they ignore you or don't pick up on the cue, you can add gentle driving pressure onto their shoulder.
If they walk forward, just block them with your hand until they can regain stillness and relaxation before asking again.