I have never actually jumped a horse. This is the basics on how you position yourself when jumping on horseback;
1.When approaching a jump sit up tall and look straight ahead over the jump.
2. Make sure your horse is in a good, even canter.
3. When you come to the jump, lean yourself forward with your weight in your heel.
This is just the basics though. When jumping, always keep your shoulders back. When actually jumping you have to keep your rear completely out of the saddle and eyes ahead. Never pull back on the reins while jumping; if a person loses his balance while jumping and catches the reins to catch himself it will tell the horse to stop. So if you lose your balance it’s better to grab onto the horse’s mane or the saddle. You always have to ride straight when approaching a jump. This helps the horse to know how tall the jump is and gives you a better take off if riding straight. Going full speed to do a jump isn’t always the best way to jump. It’s more about the horse listening to you and understanding you. That’s really important when jumping so you can change the gait to get a more even jump on a taller jump or a shorter jump.
You also don’t want to get ahead of your horse when jumping. ‘Getting ahead’ means ready too soon for the jump and going along with the jump too early before the horse is taking off for the jump. If you lean your body forward before the jump is will put too much weight towards the front and will make it much harder for the horse to jump at all. Of course you will also have to wear the right things and the gear for the horse as well.
Also, fear can make the horse feel uncomfortable and he may not jump at all. So if you're not ready to jump because you're scared, it’s better to say so before trying to jump. A horse could mess up because he senses your fear and both of you could get hurt.