Okay so doing leads is fun but can be tricky later on if done incorrectly. Chances are that you won't "ruin" her or even "mess-her up"... :) So, a round pen or ring will both work, but you have the right idea of working on a circle. Without being too extreme, it throws your horse off to pick up the right lead most time. Controversially, there is no "correct" cue combo for the canter however, ouside leg, and inside rein works really well. Make you'r outside leg concise, and don't hold the cue there. If the quick cue just speeds up a trot, then before making her canter anyway and inforcing the "rushing", just bring them back, and try again. "Inside rein" is not an actually cue, it's like a reminder and signal. Use a little bit of rein, and flick your inside wrist to force a bend to the inside. It shouldn't make your horse turn, but your horse should have some neck bend so you can see the outline of their inside nostril and eye. Also, after a few weeks/months working on canter and as your horse develops, try walk to canter transitions. You'll be able to really perfect your cues and make your horse work and develop mentally. Any Q's, PM me, and good luck!