Kayty's response is the best, to my belief. I just learned to do that on Friday and for the first time I was cantering 20 metre and 15 metre circles with no problems, and before that, I could not steer in the canter except to keep the horse on the wall. After my instructor explained it to me, much like Kayte just did, I was able to do tight circles. You don't pull back on either rein. You close the outside one low against the horses neck, much like neck reining in Western, if you've ever done that. You open the inside rein a bit - my instructor said to just "feel" it, by which he means keep it a bit loose and play the bit as needed, by opening and closing your hand. But it was the closed outside rein against the horses neck, without pulling back, that did the trick. Also not pulling back on the inside rein to turn the horse - that would turn the horse's head in front of his lead canter leg and he will break to trot. Of course, the legs and the lean, as Kayte said, but the reins that way were what changed me into a good steerer at the canter in one lesson. Good luck, it wasn't hard once it got explained to me.