My last trainer tried to get this concept going with us, but after she rode Cinny for a month, as well as I , she just threw up her hands and said...fine, ask with the outside the peanut brain will never learn differently.
The best way in the beginning to determine which leg to use is to rely on where the horse's stomach is. If it bulges on one side the hind leg is under the body on that side.
To do the alternate legs take it one step at a time.
At walk don't have much rein contact and semi close your eyes and feel where the stomach is. Just sit deep and let your leg push it back when it pushes against your leg. Let your bum cheek follow that drive on that side. I tell my ADULT students it is like playing sex with the horse. LOL
When you try it at trot DON"T go fast. Slow trot and do the same as above.
Only then take up the rein and apply a small squeeze with the left rein when your right leg drives.
The key here is to eventually get impulsion but only when you as a rider can feel the horse and are in balance with it at a gait you can cope with.
Once you can do this you have no idea of just how much you can open your horse's gait beyond what you think is possible.
You will also find it easier to do arcs and circles when trying this.