i found reading a lot and watching videos helped me, plus the odd lessons / clinics i can get. There’s an exercise I read about that suggested renvers in the canter to prepare and this helped me a lot, but the biggest help was having good eyes on the ground with a great clinic coach. Personally, the quality of canter makes all the difference for me (true supple collection, with the ability to counter flex in a relaxed way, and go forward and back, jumpy, crisp and straight) once they have the basic idea of the change.
And I like the advice I got recently (for my antsy gelding) “if he’s anticipating the change, he doesn’t get to change”. I did third with him this year and can confirm that changes “mostly there” at home will be 50/50 at best in the ring . And you won’t likely get out of the low-mid 60s without clean, confirmed changes at third.
I’m an amateur but I have shown 3rd/4th level before on horses who already knew changes. It’s not necessary but I’d say helpful to have that experience before training them on a horse that doesn’t know them.