Okey Dokey then. First practise your position, pace and timing. To get the proper position, your horse's ribcage & shoulders have to be moveable, so if he's loping on the right lead, you use a lot of left leg to move the shoulders & ribcage to right. When you actually want him to change, take off the left leg and put the right one on. Yours hands will stay straight, the lead change will happen off your legs. The pace is the speed at which your horse is comfortable with & to start, it needs to be fairly fast. The timing is knowing when to ask for the change, you have to be in rhythm with your horse to know when he can change, that's a feel thing.
Once you got that down, you can practise a countercanter exercise. Use 1/2 of the arena to start loping in a circle to the right, then cut a straight line across the diagonal & lope a 1/2 circle to the left but keep on the right lead, don't change. On the countercanter, use your right rein & left leg to keep the horse arced right. Keep him moving & don't let him change. Do it to the left & reverse the cues. When you can do this smoothly and without stopping, you are ready for the flying lead change. Do it in the center of the diagonal, if you've done your previous exercises correctly, you should get a smooth change, if not, back to the previous exercises.
Hope this helps, it's a lot of work & practise, but it's the proper way.