Leg-yielding is all to do with timing. Before trying it on your horse try it by yourseld with someone helping you. Start by walking in a straight line with your helper walking on your left. Have him/her put their hand on your ribcage . First, have him/her push gently on you every time your left foot is on the ground. You will notice that you bend around the pressure, but can continue to walk in a straight line without much trouble. Then, have your helper begin to push you gently when your right foot is on the ground. You will find that you have to step over in order to stay upright.
Now try it on your horse. Start at a rising trot. When the horses outside shoulder  is up is when you need to start your que.
Keeping your body straight, when the horses outside shoulder is up, while trotting, shift your hips to the outside as though you are going to stand in that stirrup. Your inside leg will automatically come in against the horses side. With little amount of pressure, you will cause the horse to step over.
Get the horse to take one or two steps at a leg-yield, then allow them to move straight ahead again. Make sure to praise the horse.
As the horse learns the signal, you will be able to perform a leg-yield by simply shifting your weight slightly to the outside.