Start with a tasty treat (her most favourite) at her nose in your hand so she can't get it. Lower your hand very slowly. We are aiming for an angle on the way down so you are heading for the ground between the dog's legs rather than vertically down to the ground. Lower your hand just enough for her to lower her head slightly. As soon as she does this, say "Yes/Good" and immediately open your hand to give her the treat. Then Repeat.
This time lower your hand a tiny bit further, ensuring her nose is glued to your hand. If her nose isn't there you are moving your hand too fast. When she is lowering here head with your treat, say "YES/GOOD" and reward again with the treat in your hand. Then Repeat.
As you progress closer to the ground, your dog should be folding perfectly into a drop. It is quite natural for a dogs body to fold into a drop from a stand.
This may take a few goes to get her to the ground. Don't expect it in the first session but if you get there GREAT!
There are no commands given in this exercise until she is reliably dropping all the way when you present the hand to lure her down. Once she is doing this, then you can give the drop command just before you present your hand.
If this doesn't make sense, let me know. I will try and find a video of it on youtube or something for you.