First thing is first, you actually have to teach the horse flying changes. The process is a bit lengthy to explain on the internet.. But basically you first need a horse that can do simple lead changes through the walk without the rider pulling, kicking or turning the horse into a circle. Then you also need to teach the horse how to do a balanced counter canter and from there we have something to work with. If the horse is not currently performing these movements easily, then you have to take the time to train her. Be prepared to work for a few months on this. Depending on the horse we need to then develop an exercise that is going to set the horse up for success in the changes and first practice it with our simple changes through the walk. When we have it down with simple changes, we then go through the same exercise, half halt for the downward transition but instead of finishing our half halt, we aid for the new canter lead without walking. Depending on if the exercise was appropriate, and if the rider aided and rode the change properly, the horse should perform a flying change. We never drill flying changes when we are beginning, at most I would ask for 2-3 per week for the first few weeks, or even months.
Now if you are looking for a lead swap, then you need to circle the horse in one direction, go really fast in the canter and then suddenly turn the other direction and hope first of all that the horse doesn't fall on you and second of all that it somehow ends up on the other lead. Then you have to do about 20 or so in the space of ten minutes.