How to ask for a canter depart? Inside leg is closer to the girth, outside leg PULSES behind the girth (be careful not to turn toe out). Sit up, do not lean forward. The horse can only depart on an aid when it is timed; if it is held the horse can end up crooked/haunches in.
A simple change traditionally is through walk.
Ideally when going over a pole/caveletti, do a figure 8 and jump straight, landing and turning. So as the horse takes off, new outside is already back and new inside is come forward lightly. (It is better over a caveletti or small jump.)
Also you can do canter on a figure 8, start with canter and walk say 5 strides, canter, walk 4 strides and the center, canter, walk 3 strides, until there is no walk, and you just ask for the change. That said the walk MUST be active and relaxed before asking for the second part of the circle. IF it is not, then walk (allowing bascule) until it is so.