The first thing to remember is that the correct jumping position should be comfortable for both you and your horse. You need to go with the motion of your horse over a fence, and not be ahead or behind in the movement. Ahead of the movement would interfere with his action, distract him and would often result in you being thrown backwards in the saddle. Behind the movement, and you are likely not to give with your hands, resulting in his head being jerked up, producing a poor jump or cat-leap that prevents him travelling forwards over his fence.
Because we use our jumping position for only a few seconds at a time, it can be difficult to master. Familiarise yourself by practising without jumps – get it right whilst in halt in front of an arena mirror or with a friend to monitor you, learning to hold the position for a while and maintain your balance until it is comfortable, and you can move fluently into and out of the exercise. Next, progress to doing the exercise in walk, again holding the position for a few strides. Repeat in trot and canter until your horse is familiar with your movements, and does not change his rhythm. A common mistake is to allow your lower leg to swing too far back, inadvertently encouraging your horse to speed up and/or rush. Keeping the weight into your stirrups with your heels well down should prevent this happening. It is important to maintain straightness in your position, not leaning right or left, as this would unbalance your horse’s movement.
Hopefully these tips will help you ! :)