You don’t have to be Supergirl to execute a good canter depart, but to do a great transition it would serve you well to think like her!
Tell yourself that you’re stepping ‘up’ into the canter, not forward into the canter depart.
Often riders lean forward pushing the horse down, so the horse can’t step up cleanly. Also, riders let go of the contact which drops the horse down too. So, tell yourself that you have to ask your horse UP into the canter. Think ‘Up, Up, Up’ with your upper body, hands, and leg.
Do not push with the leg like your horse is a tube of toothpaste or he will behave like one and rush out in a big splurge! With the leg scoop your horse UP under you.
If you are really struggling, lean back and ‘look to the skies’ when you ask. This temporary measure will really help you communicate the UP message, you can tone it down once you and your horse get the idea!