Heels down, with your knees not gripping to the saddle. A saddle without the support and comfort of a knee roll would help.
First you need to release your knees on flat work and be stable on your stirrups during flat work. Then when you feel confident on flat work, do it with small jumps trying to put your weight on your heels and relax your knees.
Be patient and hardworking, it will take time for this to become a reflex action and you will soon see results!
Good luck :)