There are several ways to teach your horse how to back up. One of the most popular ways is to cue your horse while sitting in the saddle. Another would be while standing, cue the horse from the front. A third way might consist of using both the first and the second manner together. Just realize that backing is an illogical movement for horses. This is not a movement they do in defense or in play. The horse backs by moving its legs in diagonal pairs, just like it does at the trot. So it has to shift its weight from side to side in order to back. The rider must apply alternate side-to-side pressures to ask the horse to shift its weight from side to side in order to back.
On a flat riding surface, ride your horse for a short time. Keep you back and hips flexible. After moving forward for a while, bring your horse to a halt and allow him to stand quietly for a short period of time. In following the first manner in cuing you horse to back, cue your horse to walk forward using your seat and legs, then gently squeeze back with your hands and the same time you cue with your legs, thus preventing the forward motion. Look straight ahead and keep your chin up. Stay relaxed and don’t stiffen up. Quietly use a voice command, “back”. Keep an even tension on the reins and don’t lift your hands. Use only a wrist motion, causing the lower part of the hand to bring the reins back. Turn then slacken your wrists, using only soft motions on the reins. Possibly a straight back pull on the reins might work, depending on the horse, but not a severe pull. Do this a few times, then move forward. Repeat the procedure until the horse understands the motion put to him.
The horse should step back with one diagonal pair of legs, and then the other. Never ask for more than a few steps. Release the tension on the reins after each practice session and let the horse move forward a few steps, before repeating.
If after working on this previous cue work, your horse still resists the cue, then work from the ground, so he can learn how to balance himself in the backward movement and understand your voice command. Never give him a reason to toss his head. If this happens, it is because he is not balanced or you could be pulling the reins too assertively. Also, the horse might turn his hind quarters. Be sure that you are cueing him with the same pressure on both sides and that your hands are even on the reins.