Slowing Down and Halt
The aids for halting a horse are a combination of legs, hand and seat. To ask the horse to halt from the walk, stiffen the lower back and at the same time squeeze gently with the legs whilst stiffening the hands and gently pulling the reins back towards your body. The pulling back on the reins should be a gentle movement and not a fierce or sudden movement. As soon as the horse has stopped relax the hands, legs and back.
If the horse does not respond immediately do not simply pull harder on the reins but relax the hands and ask again applying less leg pressure.
The same aids are used to make the horse slow down from one gait to another, eg from canter to trot, trot to walk (known as a downward transition). Remember that as soon as the horse has done what was asked to relax the hands and back.
Taken From Equine World