I can't really answer you, but I will tell you what helped me a LOT.
There's a Classical Dressage trainer named Sylvia Loch, and she said to carefully do it yourself so you can feel what muscles are going to need to activate and move, and then to apply that to the different shape a horse's body has.
So, start by standing with your legs apart, your knees bent, your hips slightly cocked, everything set up as if you were sitting on the horse.
Be warned - the first few times I tried this I fell on my butt (or nose)
Now, picture in your head exactly HOW you want the horse to move. If you want the horse to do a collected halt-to-walk transition, then you have to keep your hips cocked (like the horse needs a rotated pelvis to be collected) and you need to find out what muscles in your body need to do what in what sequence to allow you to SMOOTHLY step forward while keeping your balance AND so that you can maintain the hip position you want.
This is where I spent a lot of time falling over.
Anyhow, I learned that most of the time I needed to activate my belly and/or back muscles BEFORE I even thought of moving a leg, and that I needed to place my weight/center-of-gravity very precisely BEFORE I moved a leg, and that most of the time I shouldn't have more weight on one seat-bone, but my center-of-gravity is what should move ever-so-slightly. Oh boy, it was hard to learn how to do that
BUT once I got it clear in my head I started trying to use it on Freckle's back and WHEN I can get it right it works very well.
When I get it wrong he just flicks his ear at me or stops dead and gives me a dirty look over his shoulder