Lean back and losen up your back also practice it a lot
Knees away from the saddle, never squeeze your knees
What normally helps me the most is that i like to really extend my legs around the stomac. Make sure to also watch your equitation and it will always differ with each horse how much you lean back and feel the hors
Just sit loose and let your legs hang, but at the same time have your muscles working
One of the big tips I can give for a sitting trot would be to really get your leg underneath the horse so streatch it as far down that you can and squeeze under them. Then use your core and sit back to push your seat into the saddle, and make sure to keep those shoulders back and that core tight!