Practice this: Stand at your horse's left side with your left hand on the lead rope about 12 inches below the halter then with your right arm stretch it out toward the horse's side asking for the side-step. At first you might need to lightly touch your horse's rib cage with your right hand, but your horse will soon make the side-step with just a slight motion of your right arm. Then as you and your horse perfect the side-step add a pole,etc and practice.
This as in any lesson a horse needs to learn the same lesson on each the left and right sides. Horses have an "imaginary solid brick/concrete wall down the middle of their backs", thus meaning what they learn on one side has to be taught on the other side as well. So reverse your hand and body positions at each practice session.