Start the sweep by planting the blade as far forward as possible and pushing the blade outward, sweeping it in a broad arc, and finish with a powerful draw of the blade toward the stern of the boat. Your non-sweeping, or inside, hand stays low, moving across the deck in front of you, rather than punching out front as it does in the forward stroke. Blade orientation is important for the sweep. Twist the blade by rolling your wrist forward slightly so that the blade will sweep water away from the bow. Since most of the turning impetus occurs at the start and end of the stroke, you will eventually learn to dispense with a full sweep.
Torso rotation is very important.
It may take several sweeps, or stern draws, to turn the boat, and even this may not get you turned as fast as you'd like. Here's another trick to accelerate the turn. As you stroke, cock the boat over on its edge, toward the outside of the turn. There are two ways to lean a boat. It's risky if you don't have a strong brace.