Turning and circles
To turn a horse the rider uses the reins and legs. To turn a horse to the left, gently pull back on the left rein asking the horse to bend his neck to the left. Use your legs at the same time to encourage the horse forwards.
Placing the right leg slightly back whilst turning will encourage the horse to turn in a gentle bend rather than just to "spin" round. This is particularly important when riding a circle as this helps to keep the horse's body bent around the circle.
To turn the horse to the right the opposite aids are applied eg right rein and left leg moved back slightly.
Taken From Equine World