Ok if we're going down the flying changes path, changes don't come easily to alot of horses and if you're not sure of the footfalls in canter and in perfect balance, more than likely you'll end up with a disunited canter.
I start changes by first teaching walk-canter and canter-walk transitions. They get the horse far more clued into your aids in canter, and will lighten the forehand (flying changes are just about impossible if the horse is on the forehand). They are also used frequently in many dressage tests before you start flying changes.
Once you have walk to canters, canter to walks dowpat, start riding them with a shorter walk gap between the canters, so go canter-walk. Walk for 4 strides then canter again on the same lead. Soon you'll master them enough to get one stride of walk in between canters. When you can get one or two walk strides in, you can start having a go at changing rein (simple changes). So canter a nice big circle, keeping the canter balanced, soft and steady, then ask for a walk transition, while gently feeling your new outside rein (currently inside) and putting your new inside leg (currently outside leg) on. The walk should come immediately, change your bend and ask for canter the other way. If you're doing it well, you'll be able to get simple changes with one stride of walk between canters without a problem. THEN you can start thinking about learning flying changes ;)