After half a circle or so, switch your bend by sitting a beat to come up on the correct diagonal and applying the aids to ask her to bend around your inside leg. You may need to open your outside rein a bit if she has a tendancy to overbend or drop her inside shoulder.
Continue like this until she's responsive to changing her bend and there's no resistance or leaning on your hands. This is a great warmup for my eventer in particular, and he loves it--comes down nicely on the bit a few minutes into the exercise and really starts to drive from behind. Once you feel like she's really bending nicely to the right, apply the aids and ask for right-lead canter. If she doesn't get it, just bring her back down to trot, balance her again, and ask again. If she becomes frustrated, take a break and come back to it.
Practice your forehand and haunch turns. You may need to start them from the ground, but being able to push her haunch over is a good introduction to leg yields, which will be valuable in pushing the haunch over when you ask for canter.