You need to understand how a horse responds to pressure. To do this, simply squeeze with both legs and cluck to your horse. Do not pull back on the reins.
If you want to go left, then release the pressure of your left leg and apply pressure with your right leg and vice versa to go right. To back your horse, apply pressure with both legs after you apply pressure on the bit to encourage your horse backward.
If applying light to heavy pressure with your legs is not enough, gently thump your horse's sides. This will assist in getting more forward momentum.
