Practice 2-point on the rail trotting and cantering
tighten your core before and after jumping if you hollow your back often. hollowing your back is almost impossible if your core is tight
Try to get a nice distance from the jump and if you have a leg that swings back use spur straps and tie them to the stirrup and the girth do it learns how to stay till. And for bam hollow it in. The head make sure to never look down. The arms you want a small contact and make sure elbows are bend just about 6 inches away from withers. And if you get a short or long distance try to still sit up and make it look good
Another thing you could do is do no stirrups to strengthen your leg and core because the stronger your core the better and then you will be balanced.
Go around the school/field and practise your jumping then go onto jumps. Also get someone to record you jumping and you can see what you need to improve on:)
Before you jump practice your position while your horse is just standing checking your heels hands shoulders etc and start with small jumps. That's what I would do X