The first thing you need before attempting leg yield is a good, straight forward-going walk.
You can turn down the three-quarter line of the school and use the horse's natural inclination to return back to the wall or side of the arena to help you get started.
So after turning down the three-quarter line your inside rein flexes the horse to the inside. Ask the horse to flex by gently curling and uncurling your inside hand around the reins until the horse flexes. This is all that should be needed. Do not pull back on the reins and try to pull his head around - not only will the horse resent it, you are likely to cause too much bend in the neck and you'll never get a good leg yield from there. You should have enough bend when you can just see the eye and nostril.
Use your inside leg a little behind the girth in time with the stride. It's important to use the leg in the press-relax-press-relax way - a constant pressure will cause the horse to lean against your leg and not move away from it. To time your leg aid you need to press as you feel the horse's belly swing away from your inside leg. This is the point when the horse's inside rear leg is coming underneath for the next step forward and is the only point you can effectively influence the horse's stride.
The outside rein is held against the neck but doesn't do anything actively. The horse will feel the outside rein against its neck and it stops him from falling out through this shoulder and diving back to the outer track which is another common problem.
The outside leg doesn't move from your normal riding position but can be used to ask for more forward impulsion if the walk begins to fade away.
thankyou, i knew the curly hand bit but had forgotten about the leg and belly bit!
Its about 5 years since i first tried it- and yesterday, although i did it properly (hacked out with my friend, who couldn't explain it to me) i just felt a bit scruffy with me over-exagerated leg aid!