Tons of practice!! And as the others have said, no stirrups. After you understand what you need to do in the canter, loose the stirrups! It forces you to actually sit. At first I was afraid so I started by picking up the canter witt the stirrups and losing them in the canter. I was afraid of bouncing off during a fast trot to pick up the canter before/after, but using the stirrups before and then only not having them dying and after made it a ton easier for me. You also will need to work a TON in the sitting trot! It helps you get a better seat and absorb the motion of your horse. I did, oh goodness, sooo much sitting trot before I was allowed to canter at my barn. Sitting trot is a fundamental.
get your coach or experienced fellow rider to lunge you're horse while you're on him, put a knot in your reins or hold them with one hand then grab the pommel with your inside hand and lean back, try to really get a feel for your horses movement and stride. eventually you can let go of the pommel, then when you are still comfortably sitting your canter (leaning a little back) sit properly- keeping in mind the rhythm you previously found. after a few times practicing this, remove you're stirrups so really get that deep seat so you can stick to your saddle. hope this helps!