you need to get them doing walk canter transitions first, then at a set point on a circle (some people use a pole but its not recommended) go from canter to walk say 4 steps then ask for canter on the opposite lead ie true canter walk counter canter. and you gradually reduce the ammount of walk steps in between true/counter canter
another way is to position a pole at an angle across the end of the diagonal, then come across in true canter so you meet the track at an angle at the pole, just before the corner. most horses will use the pole and the extra lift gives them time to do a change. the canter needs to be good, balanced, uphill etc first though, obviously.
I teach from Half Pass - once half pass established half pass from Centre Line to track right in corner and instead of asking for continued counter canter, swap legs round and ask horse to step through with opposite hind leg.
Personally I find the decreasing walk steps encourages them to anticipate simple changes and you end with a choppy change
I was taught to do it over the diagonal and my horse anticipates it each time we canter on the diagonal, saying that Ive tried half pass too (to be fair we havent quite got it yet!) and he changes automatically then. He anticipates most things though so I have to keep varying when and where we try moves.
That's why I use HP - can then continue with counter canter or ask for change - make sure you're not inadvertently moving your body weight across the diagonal - used to throw my boy over loads and I really had to think sit left, sit left, sit left with every stride