Hi, I don't know if I will be able to explain it with words but I will try aha!
You must first get them soft in the face so that when you pick up your hand and make light contact with the mouth, they will lower their head and break at the poll. As they become more accustom to carrying their head in the correct positon, you simply keep "fixing their face", you put their head where you want it, and when they move it up, you bring it back down. Over time they figure out where to leave it as they avoid pressure by leaving it in the correct spot.
When the head comes up, apply rein pressure, and only release when the head goes back to the desired position. You have to make the wrong thing (rising the head) uncomfortable by applying rein pressure and the correct thing (lowering the head) comfortable by releasing the rein pressure.