I find telling people to pretend they're sitting on a big fluffy marshmallow; squishy enough to relax but you still have to hold yourself up or you'll fall off.
Allow yourself to move with the horse. A canter is bouncy but don't tense against it. Focus on your breathing and keeping the weight into your feet. There's a lot of things to think about to perfect the canter but I find that the biggest problem people have when they start to canter is tensing. Taking big deep deliberate breaths and imagine yourself just sitting on that marshmallow, using your stomach muscles and weight in your feet to keep you upright. The rest can be worried about after you're feeling more balanced and secure.
The way I learned is many ways. Keep these things in mind: the canter is a three beat gait that goes in kind of a scooping motion. This will help mentally.
Try to keep your legs still and forward the best you can and keep your heels down. Let your hips relax and roll in the saddle and follow the horse’s movement. Keeping a soft but straight back will help a lot. Try not to lean back; that’s what I struggled with. Lean forward (obviously not too much) and keep the horse moving. Leaning back will push the horse forward.
Try not to rely on the reins. Keep your hands steady and while keeping short reins, make sure you give the horse neck to stretch. Don’t pull up or make any harsh movements. Keep your hands down and not to tight. Give your horse room to run.
An alternative for learning is having someone lunge you. This gives the horse a canter but still control and slow. You can take your feet out of the stirrups and focus keeping your seat the way I described above.