Put ground poles on the diagonal to where you can do a constant figure-8. Then canter as you normally would, push before the take off (to get more jump) and slightly ask for more contact on inside rein over the jump. I you mess up, just change it through the trot. I recommend doing this about 15-20 times the first day, then only a few in the following days. In about a week, take one pole away to ask for the change, instead of jumping. If it doesn’t work, just continue with both poles. Especially with young horses, never let them get away with doing it wrong, they get very habitual. Hope this helps!
Start by an open arena with nothing in your way. Lay one pole in the center. Keep an upward and collected canter and do right lead over the pole. Then trot and do left lead a couple times. Then, work on the changes! Start on one side and then as you go over pole, give the cue (pull inside rein and out side leg). If they don’t get it, don’t stress it and kindly have them trot and pick it up correctly. I’ve also noticed that after a jump, if my horse is on the wrong lead and going really fast, I tend to just give the cue, but if I collect him up, he can get it.