Everything that everybody else said lol but also what I like to do is slightly bob my heels up and down like i do in the sitting trot. It's like pushing your weight into your heels every time the horse's stride moves downward if that makes sense. It helps you sit the trot and the canter:)
Think of heavy objects such as a Giant concrete box, or a dishwasher it may seem weird but it works for me 😂
Put your heels down, relax your knees and slightly open them. Remeber to sit and slide going with your hips forward and let yourself feel the horse and find their rhythm. Eventually you will feel how to sit and it will come to you! (Lots of practice helps!!),
I agree with my fellow riders below and just want to add one thing
Relax your hands
Practice in the sitting trot and then try in canter. Attempt to move with him.
breathe out, push your heels down and make sure you have legs😊 good luck!
Push your heals down and keep your core stronger 😉
Push ur heals down, knees down and keep ur core strong😊 Hope this helps xx