i would start by coming down the 3/4 track and pushing the horse back on to the outside track. just remember that the horse has to lead with his forehand, and the quatures follow. you see so many people leg yielding with the horse wrapped round their inside leg. with leg yield, your aim is to get the horse to move away from your leg.
Push the horse away from your leg, slight bend away from direction. must keep horse forward going as much as possible. i.e its not a half pass that is trying to be achieved !!
It also kind of depends on whether the horse already knows the movement. If not then you can help them get the idea by slightly overexaggerating intially. Everyone teaches their horse differently but this is what I have always done:
Turn down the 3/4 line straighten the horse up, then take a very fractional (so you can just the the corner of the eye) opposite bend to the direction you are going in and support this rein. Then open your outside rein to show the space you want the horse to move into and apply your inside leg at the girth, if the horse does not move over you can try placing a whip down the shoulder and pushing into the shoulder (don't smack). Usually horses have been used to people pushing on the shoulder to move them over when mucking out so this trick often gets the movement started. If you are still struggling get a set of poles out down the 3/4 and have them slightly angling in gradually towards the track for further encouragement.
Once you have the movement started it is a case of controling the speed of the shoulder and quarters. Close your outside hand if the horse starts to run through the shoulder. If the quarters come over too quick then I apply my outside leg behind the girth and raise my inside hand. Basically throughout the movement your hands and legs are trying to maintain the correct angle and you will find yourself opening and closing your outside hand, using your inside leg for over, your outside leg at the girth for forwards. Initially go for a fluid forward movement rather than attempting too accute an angle this way the horse does not get in the habit of being stuffy in the movement which is harder to fix later.
If the horse is new to it only do a few steps and reward then go straight and ask again. Build it up over a few sessions do not go at it too much as the horse can start to become stressed and then they tend to start swinging their back end around which is not productive.
ETA: once you have the hang of it the art is to tone down the aids and keep the impulsion...