This video isn't bad, but I would like to see the horse soften its face a little more and really march into the circle, but you can get the basic idea from this.
Also, I prefer to do it at the walk or the trot, but every now and then I will do them at the canter too just to be sure my horse is stil listening. So if I had him on the left lead, I would circle to the left but bend him to the right.
You could always start your ride like this too, and establish that control before you even start work. What I'll do is I start with small circles, asking for a lot of soften and bend just to get the horses loosened up. Then I'll keep that bend and counter arc them away. Then I might change the bend but still keep the circle, or change the circle and keep the bend, or change bend and change circle at the same time...Really anything. Just something to keep them guessing and thinking, and stretching out. It's as much a technique for the brain as it is the body.