Direct reining most horses is fairly straightforward. This article discusses 1. teaching a horse to direct rein and 2. how to direct rein a trained horse.
Teaching: The best way to teach direct reining on a green horse is through ground driving (see article index for information on how to train a horse to ground drive). Grounddriving familiarizes the horse with the basic direct rein aids. You wouldn't drive a car that didn't have a steering wheel installed- don't ride a horse that hasn't been taught to steer.
Riding: Riding a horse trained to direct rein is usually fairly simple. Mount and grasp one rein in each hand, each at an even length. The most elementary horse should respond to a tightening of the right rein to turn right, or left rein to turn left.
You should never need to pull a rein to turn, this will create a hard mouthed horse that becomes increasingly difficult to turn. Instead, you should give direct rein aids as if you were squeezing a lemon: Instead of pulling, simply tighten your fingers gently on the rein in the direction you want to turn.
A horse trained by a more advanced rider might find rein aids alone annoying. For this type of horse you should use your entire body as an aid in addition to your reins. Instead of squeezing one rein gently, you should 1. turn your upper body and LOOK in the direction you want to go. 2. apply pressure with the opposite leg, and 3. gently apply the rein aid. Horses are sensitive enough to feel a fly land on their hide, if ridden consistently and kindly any horse can be taught to turn with the slightest aid of your seat.
A note about direct reining: Direct reining should NEVER be done in a curb bit or any bit with shanks. Curb bits are designed for neck reining and are not suitable for direct reining. Curb bits are so sensitive they are designed to be activated by the slightest movement in a slack rein- to apply direct pressure on a curb rein applies pain in the mouth, chin, and the top of the dead. Please be sure that when you are direct reining you only use a type of snaffle bit.