How to get a flowing soft seat in canter?

Doing no stirrups at the canter! It really helps. It forces you to move with the rhythm of the horse and challenges your balance
Something that could help you is while keeping julia and katelyns points in mind. Having someone else lunge your horse while you ride. It may sound silly as something you usually just do with beginners, but it allows you to focus solely on your position, and what you are doing. You will be able to hold the saddle and feel how your seat is naturally tilted while at the canter. You can then actively make those movements, not being afraid to exaggerate. From explicitly making those movements, they will eventually come naturally over time, just like posting or following with your seat at the walk.
Just relax and try to move your hips with the horse's motion. Try to not grip with your knees
- dont lean
- knees away from the saddle
- sit up
- relax - this may take practice as balancing is not easy at first

honestly i had this exact problem for a long time but the only thing that fixed it was putting the horse on the bit, dressage training and practiceeee