A fairly good way to guage proper jumping position, when you don't have anyone around to help you from the ground, is first shorten your stirrups so that your leg is at almost a 90 degree angle when seated. Then at the halt, Stand up in your stirrups, straight up so your aligned top to bottom, head to toe with legs at the girth. Then with the idea of keeping everything from your knee down exactly where it is, shift your upper body back down into half seat position. You'll probably have to play with your position a bit until your shoulders are centered over your legs. You'll most likely find that it takes a ton of thigh and lower back muscles to keep you in position. Once you find the correct spot, practice holding it at W/T/C. Then move onto ground poles, crossrails, etc.