Awareness: The First Essential Ingredient
Draw a set of train tracks in the sand about 3 feet apart and in the shape of a circle about 20 feet in diameter. Try to ride your horse at a walk between those train tracks but only use the inside rein. Of course, you will use your seat and legs, but don't over do it by using them to try and make the horse stay between the tracks. It won't work. Don't use the outside rein at all unless you get completely out of the tracks or you need it to avoid a wreck. This is one of the best ways I've found to help people start to learn what "feel" is and become aware of how inappropriately most riders depend on and use the outside rein to pull on the horse.
To be successful, the rider must maintain very good balance and help the horse to do the same. The rider must feel and think about what the horse is about to do or where he's about to go. If he feels like he's about to drift across the outside track then the rider must direct the horse back between the tracks before he crosses it by using the rein to step the inside front foot toward the inside track as it leaves the ground. If he feels like he's about to cross the inside track then rider would use the inside rein against the horse's neck without crossing his mane to step his outside front foot toward the outside rail as that foot leaves the ground. In both examples, the rider must release by softening at just the right moment.
If at any time the rider pulls on the horse he will assuredly go too far. Throw him away by not directing him with the inside rein, and he'll wander out of the circle. Hold on for a split second too long and he'll travel too far. You must release at just the right moment by softening. If he traveled too far, you didn't necessarily do too much, but you definitely didn't release by softening soon enough. Remember, the horse learns on the release. You must feel when he is about to go where you'd like and release by softening but do not throw him away.
The tracks are a good visual to use because it allows a person to see where the horse is going and if he needs some direction to stay between the tracks or if he needs to be left alone to continue doing what he's doing. It really helps the rider to become very aware of where they are with their horse and where they are about to be. I believe this awareness is the first essential ingredient in learning feel.