Hi! Firstly, relax. :) take time to take a few deep breaths and relax every muscle in your body.
Ok, the jump position is all about the fold from the hips, you should take the weight into your stirrups and then really fold from the hips, pushing your bum towards the back of the saddle.
Imagine the weight sinking down into your heel. You may want to shorten your stirrups a hole looking at the photo. Be careful that your toe doesn't sneak in front of your knee once your stirrups are a little shorter. The shorter stirrup allows you to fold easier.
Keep the upper body straight but lower your shoulders a little. You want to flex and fold rather than tip. Your centre of gravity needs to remain constant and by tipping you will become unbalanced. Keep the arms relaxed imagine they are elastic and will follow your horses movement.
Practise the jump position at halt and once you are balanced, try it at walk, then trot, then canter and then over trotting poles. It is invaluable doing work in the jump position without using fences. This will strengthen your position and balance before you even go over a jump. You can also practise the jump position when working without stirrups as this will improve balance, strength, core stability and stamina.