Before I say anything else, I want to advise you not to use a tie down, or any contraption similar. All that teaches them is to brace, and they never learn self carriage for an independent headset, and their muscles develop in every way that is wrong for WP or hunters, or whatever it is you do.
Now, the headset, is coming only from pressure and release. A lift on the reins, and a squeeze with the leg, and then hold until they drop their head. Continue that. It'll become faster, and softer as they understand it more, and their muscles develop for it. Start at the standstill, once that's mastered, then move up to the walk, etc. With more muscle, they'll become more able to hold the headset longer, until eventually that's how they'll hold it always.
The slow pace, takes a lot of hind end impulsion. There's so many ways to achieve this, but a lot of transitions, backing, circles, to build up the hind end so it's easier for them to settle in a slower pace. After that, it's all seat and energy, and you have to ride the pace. Horses like rhythm, and they will match you, if you set the pace.
Only work the pace though, after you have the headset corrected, and the horse can move independently through their rib cage and topline.
"You're just as sane as I am."~Luna Lovegood.