Part of it is timing. You want the horse to be able to step out of the canter and into the trot. The first beat of the canter stride is the hind leg striking off. The beat before that is the moment of suspension. If you still your seat and half halt during that moment, your horse should downshift into trot with that hind leg instead of continuing to canter. Make sure you're sitting square and tall with your core engaged so you don't fall forward. I'm sure the more experienced dressage gals will have more to add. That's all I've got.