Practice your two point during a different movement. Like the trot. Once you have that down just repeat it during the jump
Bend at the hip not the knee
Break at the hips so it's more fluid
Make sure I turn your knee out starting from the hip joint so that when you land you can take the impact and land with flexible knees opposed to making legs straight so you can't actually move with the horses natural canter
I wouldn't recommend gripping, gripping can make your horse crazy after the jump and altogether is a bad habit.
Don't forget to grip with your lower leg, one I started this it fixed my jumping position so much. If you grip with your lower legs and keep your heels down then it's harder stand up in your stirrups, you have to rather fold over the jump
You may need to change your stirrup length
Just remember to Fold at the hips. Practice your jumping position on the flat, over ground poles, and small jumps. Make sure your stirrups are the correct length too
Drop your irons and practice small jumps feel how your seatbones stay with the saddle at all times when you get your irons back keep that same feeling with your seat bones
Practice your jumping position on the flat before you jump. Your bum should be barely out of the saddle, and you should fold at the waist like a hinge. Your leg position shouldn't change, and as you're doing the jump your hands should give a little to allow the horse's neck stretch out.
Push your heels down and lean forward, allowing the horse to come to you.