To ask a horse to walk you squeeze gently with your lower legs and once the horse starts to move forward your legs should be relaxed. Some horses may not be as responsive as others and may need a small, gentle kick with both heels to move foward. If, whilst walking, the horse starts to falter a soft squeeze with the lower legs will persuade it to continue.
The walk is a "four step gait". This means that the horse moves each leg in turn and places all four legs on the ground individually.
As the horse walks you will feel the body sway from side to side. This happens because as each hind leg in turn is lifted by the horse and moved forward that side of the horse becomes higher. And so by feeling the sway of the horse's body from side to side you can tell which back leg is being lifted and is moving forward.
As the horse walks you will also see that it moves its head backwards and forwards in a nodding motion. To allow for this you should keep your arms relaxed so that you can allow your hands to move forwards and back with the movement of the horse's head. By moving your hands with the movement of the horse's head you can maintain the same tightness of rein (known as "contact") throughout without restricting the horse's natural movement.
Taken From Equine World