Leg-yield to a circle is a suppling exercise that can help to increase your horse's attention, obedience, ability to bend and engage inside hind. For the rider it is a great exercise in focus, proper application of aids without disturbing combined balance and planning ahead to create a smooth blend of two different movements. The great thing about this exercise is you can ride a numerous combinations of it. From very simple to quite complicated ones that involve several changes of bend and direction. This exercise in its more difficult form is introduced in First Level test 3.

Now I will list the variations in a descending order from simple to difficult:

• Leg-yield from quarter line to a wall followed by 15 m circle - This one is the easiest because the arena is used to favor horse's desire to move toward the wall. The bend does not change throughout the exercise and diameter of the turns and circles is large. You can repeat the exercise on each quarter line or use the same location and allow your horse to have a mental break on the following long side. Riding forward and refreshing the trot is a good idea.
• Leg-yield from centerline to a wall followed by 10 m circle - This one is the similar to above. Diameter of turns and circles is smaller which increases the difficulty. And the leg-yield is more acute with greater sideway movement, especially, if you do not have a big arena. Refreshing with the forward trot either on a diagonal or long side is more important to prevent your horse from losing push from behind.
• Leg-yield from the wall to quarter line followed by 15 m circle - This looks similar to the very first one but the difficult part comes from the fact that you must leave the wall for a leg-yield and flexion/bend will change twice: first after the corner into leg-yield and then from a leg-yield to a circle
• Leg-yield from the wall to center line followed by 10 m circle either direction - Here we see increased bending, increased sideway movement and increased bend. To ride this exercise well you need more engagement and obedience from your horse and you need to stay in the moment to prepare the next movement.
• Leg-yield from the wall to center line followed by 10 m figure eight in the center and a leg-yield back to the same wall (First Level Test 3) - I like this particular exercise and was very happy to see it in the new dressage tests. I think it is a neat combination of movements that require horse and rider to really work together, stay balanced and straight. If you are not sure you can execute it in trot try it in walk first. It will give you a taste of the flow and transitions between the movements without complications of trot where everything comes at you much faster.
• Leg-yield from one quarter line to the other followed by 10 m circle - I put this one last because all the movements are done away from the wall and this increases the difficulty level because you are only left with your skills, no walls will help you to turn your horse, stop him from drifting, or stop him from making circle bigger than you want. It is all in your hands. Not literally, though!

I didn't give very specific descriptions on where exactly to start a leg-yield or where to ride a circle. This is another way to create even more variations. Depending on your horse specific areas that require improvement you can tailor these exercises to your own needs. The principle here is to ask your horse to do opposite of what he is prefer on his own. For example, if he is rushing leg-yield - slow him down or start circle earlier, if he is cutting into a circle - spiral out, or delay your circle, etc.
You can make your horse leg-yield faster and start circle earlier. Or you can postpone the beginning and start leg-yielding later. You can ride this exercise in slower trot, faster trot, more forward trot, more collected trot, or a combination of two. My only warning is to plan ahead and then work on executing your plan. This will help you develop discipline and more accurate riding. If you find yourself creating a more difficult exercise than you and your horse can handle, recognize that and simplify things. It is important to challenge yourself and your horse, but not too much to make it too hard for both of you. Remember you can increase a level of difficulty every ride by very little and before you know your horse will be executing movements with ease and precision. One more thing to remember - do not drill! Repetition is important, however, stop before your horse is tired and/or bored.

Happy riding...