Most generically-correct answer is "you get the horse very well balanced, and then as you change bend (it is easiest for the horse if you do this at the center point of a LARGE figure-eight) you apply the canter aids for the opposite lead at the right moment." If the horse is used to going over a pole on the ground, that can help too but then it requires you to be even more precise in balancing/bending/steering.
However this relies on your knowing when the horse IS correctly balanced and bent, and what the correct instant in the canter stride IS to apply the aids, and how to use your aids to COMPLEMENT what the horse is doing instead of accidentally getting in his way. It is awfully awfully easy, even in a regular ol' canter depart let alone in a flying change, to be intentionally or accidentally leaning/flapping/swaying your body around in a way that makes it unappealing or impossible for the horse to do what you want him to.
So really the most sensible answer for someone asking the question on the internet is "get a trainer to help you". Really really.
Pat