Canter - walk should be ridden more through the seat, than by pulling on the reins. Pulling on the reins will dump the horse straight on the forehand, pulling them off balance and making the transition more difficult.
You want to imagine that the horse is 'sinking' the quarters into walk. The transition shouldn't be visable, the horse is cantering one moment and walking the next.
I ride canter-walks with seat only. The horse should be well balanced in canter, working in or towards collection. When you want to come back to walk, think of creating a 'canter pirouette' canter, almost bringing it on the spot. Imagine that you are nailing yourself to the ground in this moment, sit deep, engage your core and think halt. Your core should be burning!
If you get those god awful unbalanced, jack hammer trot strides in between, pick up canter again, refresh the quality of the pace, and try again.