That sounds about right the way you're doing it. I trained my mini to bow the same way but he was small enough for me to put pressure on his withers as well... Another way to do it is to get a nice soft, cotton rope (one that wont tighten too much) & tie it around the horse's foot & pull up with the rope, that way you have more leverage & you're in a better position to apply pressure to her halter in case she tries to pull her foot out... it's hard to explain
My friend & I were teaching our weanlings to lay down the other winter & we used a cotton catch rope to hold their front leg up while we pulled their heads around to the other side (away from us) Theoretically
this is supposed to set them off balance & they'll just plop over - not really! It takes a bit more persuasion than that at first.
Here are some pics of bowing & the one filly laying down. (Pally's first time bowing too)