Answer

By Your Horse

Dressage Rider and Trainer Claire Lilley replies:

To introduce half-pass, leg yield to your left across the diagonal in a collected walk until the centre point, then stop. Your horse’s forehead should be facing onto the other diagonal, towards the right, and you should look that way.
Keeping your position exactly as it is, ask with your outside leg for the horse to step sideways to the right (in the direction he is bending) and keep the flexion with a firmer inside rein. The outside rein should soften slightly to allow the horse to travel in the new direction, but if you let your outside rein go too much, your horse may overdo it and lead with his haunches.
Walk slowly along the diagonal, half-halting to rebalance your horse as you need to.