Little by little. You put your hand on the poll and press downward while at the same time pulling down on the halter via the leadline, and when she gives a tiny bit you stop all pressure. The trick is to NOT give up the pressure for all incorrect reponses; I.e. If she responds by jerking her head up, you follow her with your hand and keep the pressure on. If she pulls away, you follow and keep the pressure on. Doe'st have to be much pressure , but enough to cause her to search for a way out of the pressure. When she puts her head down, you release. All the way.
Once she starts to get it, add the word "Down" .
As she gets softer, you release only 50% and then you ask for a little bit further, then total realease. You keep adding to the amount that you are asking her to lower.
You can also encourage her to go down , all the way to the ground, by putting a cookie on the ground, but only after she has learned to respond to pressure on the poll and the lead. If she gets really soft, you can just use the lead lind pulling down.
Just take it in small incrememts and reward for small tries.