Okay, teaching hips in will help your horse learn to move her hip over when you cue her. Teaching her to move her hip over when you cue her will help her pick up her leads properly. When you ask for a canter depart, you will first use hips in to set her up, and then ask her to depart. In time, she will quickly adjust and take off with the correct lead with just your leg position. When her hip is in the correct position in the first place, she will lead with the correct leg in the back, which will drive the front into the correct position naturally.
Begin asking for hips in by bending her to the curve of a circle. When her head is soft and flexed in, pick up the outside rein (but not enought to bend to the outside - you just want the touch of outside to be associated with the hip going in). After making contact with the outside rein, keep you inside leg on the girth and slide your outside leg back behind the girth. Next add pressure with the outside leg to say "get that hip over" (it will help to play porcupine game with that area first from the ground so that she will want to easily move from your leg pressure). Next, if she does not make any attempt at moving her hip over, drive with both the outside and inside leg at the same time, and keep contact with the bit (with head still slightly inside). Drive until the hip moves over, even a little. Her head should stay soft and responsive, and in a desirable position...keeping contact while driving will help that.
When she does hips in at first she will want to stop. That is okay, just encourage her to walk back out. Eventually you can do it at the trot, and again she will slow, but again encourage her to trot back out. When she gets it down, use hips in to pick up the correct lead...bend head into direction of lead slightly, ask for hip to come in, ask for canter depart, hold leg position for additional support while cantering if necessary (at first....eventually she should be able to balance herself).
Here is what hips in looks like at the walk...sorry for image quality :)
At some points he is four tracking, which is a little extreme, three tracking would still get the idea of "move your hip over" okay too...but he is still learning :) :) In the first part where we are walking away, I was actually cueing him to move his hips to the left, but as you can see, he went the opposite direction at first....I kept persisting, and he fixed it though. Teaching is is helpful, but you have to be very exact with your cues...there is some confusion in communication early on, but it makes a world of difference. When I was first really teaching it, he would try to give me extreme hips in when I was asking for a canter depart....oops...you just have to cue very precisely (canter depart for him is hips in position with actual drive from the inside leg, but he would just think I wanted his hip in farther :) )