People posting here have some really great ideas. I didn't read everything so sorry if I mention something that's already been said:
*Lots of no stirrup work
*Two-point until you can't two-point anymore
*One exercise that my instructor had us do recently was to post the trot without stirrups and then go into two-point over three ground poles. I think doing that a few times would help build your muscles and help your form without the pressure of actually jumping.
*Do something you can easily do and then just practice everything about jumping over it. Your approach, counting strides, your feel, your form over the fence, etc. Then you can take what you learn with that and apply it to more advanced things.
*Watch videos of other people jumping.
*Go into the "riding critique" section of this forum and watch videos of people jumping. Then read the comments to see what people said about their ride. After a while try to pick things out and see if you were right.
Jumping is tricky. I bet in a while you will look back and say, "Wow, I ride so much better now, I can't believe I ever rode like that!"