All I know is that when I asked my horse to canter, my inside leg was at the girth. My outside leg was a little behind the girth. I squeezed both, and drove with my seat and he picked it up on the correct lead and kept it until I put more weight down in my legs.
Now I don't know if that's necessarily the correct way, but having my outside leg behind that girth helped him to keep his hind end behind him rather than swinging out.. and having my inside leg at the girth served two purposes: driving him forward along with my seat, and keeping him from dumping into the inside.
All I know is that it worked :P!