Walking. walking walking walking and more walking, and then a bit more walking. And some walking up hills, some walking down hills, walking over varying terrain. Then some leg yield in walk on both reins, walking walking.
A month of walking 5 days a week and you can introduce short bursts of trot, up hills is best ;) Start working on leg yield on both reins, and millions of walk-trot-walk-trot-halt-trot transitions and changes of rein.
The key is forward into an elastic rein, allowing the horse to reach into the bridle and travel off the forehand, rather than just dragging itself along on its front legs and bracing it's neck .