A lot of ground polens and gymnastic exercises. If you Google you will find really good exercises. Other wise i recommend you to speak with a good trainer nearby and take help from them.
ground poles. play around with spacing and stuff. oxers are great as well imo
I recoomend you to put a pole in the ground at the front and back of the jump with a distance of 3 mts from the jump to improve the technique
To improve your horse's scope you need to focus on teaching the horse to use his body correctly over the jumps. So to make sure he is rounding over the jumps and coming up through the wither. For this I would recommend lots of gymnastic work. Lines of one and two strides. Also exercises like three trotting poles into a deep cross. If you can do this exercise on a slight downhill its eveb vetter as it forces the horse to sit down and use his hindquarter. I would also reccommend implementing a hill work session. Walking and trottong up and down hill in a frame.