The leg yield can be done along the side of the arena, across the diagonal, on a circle etc. Just about anywhere! The body is straight, while the poll is flexed slightly in the opposite direction of travel, with the inside hind and fore crossing over the outside hind and fore at the same time. Theoretically, the leg yield is actually NOT a lateral movement as there is no bend involved. However it is an effective lead up to lateral movements as the horse is moving away from the leg laterally rather than just moving forward, and teaches the horse to accept, and work into, an outside rein contact.
Problems that can arise in leg yield are the horse moving too steeply across to the track, so it basically goes straight sideways! This is combatted by riding forward in a straight line every few strides, half halting, and asking for a few steps of leg yield until you reach the end of the arena. Another issue is also the horse running on when you put your inside leg on. Again, the combat this, half halt (or give a half halt aid even if the horse is not experienced enough to able to fully respond to a correctly applied half halt) to get horse back with your seat and rebalanced, then ask again. Exaggerate the half halt so the horse almost walks, if the horse is being particularly pushy about running on.
Another issue, is the horse just walking straight on a diagonal line. Many beginners at leg yield will be fooled into thinking their horse is in leg yield. Make sure that your hips and shoulders are parallel to the short side of the arena, look ahead and push the horse across with your inside leg.
The aids for leg yield, when riding a more experienced horse are:
Weight your inside stirrup
'Feel' your inside rein to ask for a little flexion
Maintain a firm but elastic outside rein
Keep your hips and shoulders parallel to the short side wall
Apply inside leg on the girth
Outside leg slightly behind girth to support
A horse that has not been taught leg yield however, will need a little more encouragement.
The main change I use when starting with a green horse, to to open my outside rein right off the horse's neck to 'open' the outside of the horse's body. Contact on the outside rein must be maintained of course, but out away from the body to free to outside up a little. Then I'll apply the same aids, weight on the inside stirrup, inside leg on, 'feel' the inside rein and away we go ;)
It is easier to teach basic leg yield steps on a large circle. Start on a 20m circle and once the horse is travelling comfortably, ride the circle a little smaller, to about 17/18m. Apply your leg yield aids to get back out onto the larger circle.
You can soon start riding your leg yield from the circle onto a straight line. So go back to your 17m circle, and just before you get to the long side, start asking for leg yield. Instead of continuing on the circle, ride large around the arena, leg yielding back to the track from the circle.
Starting with only a couple of metres off the track will give the horse, and you, more confidence. Then once you are both competent at leg yielding a couple of metres, you can start your leg yield closer to the centre line. Ideally, you want to have enough control of your horse, to be able to ride leg yield zig zags. It took my ottb only one week to be able to leg yield OFF the outside track onto the centre line, then back from the centre line to the outside track. He is now able to ride a zig zag up the arena incorporating 3 changes of rein - not bad for a very green ott!
Any horse can do it ;) And you'll be pleasantly surprised at how much more reactive he'll become once you've mastered the leg yield.