It all stats with the seat! sit the last few strides before the canter. Use your seat to draw up the horse's hind legs, asking for more impulsion. The inside leg has a very important job in this moment. If your horse has a tendency to lean in just before the transition, your inside leg becomes even more critical in helping the horse maintain balance by not allowing him to drop his rib cage toward the middle of the ring. The outside leg initiates the lead. Some people call it a "windshield wiper" motion: swing your lower leg behind the girth to ask for the first stride. The horse's outside hind leg should strike off into the lead as your leg reaches back.So far, your seat should have been trotting. Now, it needs to transition. So you go from two seat bones moving in tandem with the horse in the trot, to a canter motion with the inside seat bone leading (to allow for the horse to take the inside lead). Your seat now needs to promote the canter movement - swinging back and forth thanks to your supple lower back.
Keep your shoulders fairly still by moving through your back. The swinging movement allows for the illusion of your shoulders staying still while the horse is moving.