Trot and canter transitions for days. Get a nice round trot, sit up, canter, one or two strides, back to trot, set up, canter and repeat. May take a couple weeks but soon as your horse stays round (over his back) canter on for several strides and then reward by doing something different or finishing the ride.
Lots of transitions from walk to trot and back and from trot to canter. And as always, ride leg to hand, you can’t have contact without leg and the impulsion for a good transition comes from the back end.
I'd have a good contact with the outside rein and flex/half halt with the inside rein as to get his attention and also make sure you have a nice forward trot before you ask for the transition as if not your horse will be heavy on the contact a resist your aids
Try to sit deeper, keep a strong contact with the mouth (not pulling), it may be a good exercise to do a circle before the transition on the sitting trot. If the horse was of the bit while transition, then go back to from what you did it and try again until it works (if you really want to make it work, better don't continue trot or canter if the transition was bad). Good luck =>