Does the horse yield on the ground? If not, you may need to start there.
Under saddle use a wall or fence as a training partner. Stand next to the wall and ask the horse to move over - either front or hind - and complete the arc. If the horse steps forward or back - correct.
ONE step at a time so the horse understands. Focus on where your legs, feet and hands are as you communicate with the horse. Use one rein to indicate direction and the other to block.
I also like to set ground poles in a grid to work on lateral exercises. If the rider has objects to focus on, it can help relate that focus to the horse.