Using poles helped me a lot. I'm working with a 16y/o now and he's gotten a lot better because of poles. If you put them out from the corners (so you have enough room to turn) you can go towards the corner canter over and ask for the lead that way. When I'm heading to the pole I try to think about shifting all my weight to the outside, then over the pole, shifting it all to the inside and asking for the change
When I teach my older horses or lazy horses I make sure they have the canter transition perfect before anything else and make sure that that are doing the transition exactly when I ask when I’ve done that I start going across tha diagonal and do a canter to trot then as soon as my horse and I are ready go into canter in the opposite leg as my horse understands this I add less trot in then eventually you can do it with no trot for me this method has worked the best out of all of them but feel free to explore more methods!
Hope this helps