I simply do it the classical way. You raise your hands up towards the horse's ears so there is equal vertical tension in the reins. You hold there. As soon as she pushes on the bit you release. Repeat. Wait for her to drop her head by pushing on the bit and release. Repeat. You do this until you raise your hands up and she puts her head down. Once she does give her complete freedom and only correct her once she brings her head up again. Aim to have her nose on the ground. She basically learns to push against your hand when you lift them. While you ride you lift your hands and she will stretch down.
If she leans too hard on he bit you hold one hand still and pretend you are shaking salt on something creating a vibration on the bit. When she lightens up you stop. 👍🏻
You first have to have them pushing forward into contact and using their hind end. Then for every centimetre of rein that you let out, you have to apply more leg so that they keep their forward momentum and keep working their back muscles.