Just adding some tips to what has been said: I start ours over a log if one is handy so they get used to being "honest" and not cheating. Ours all LOVE to jump but I do have a couple who will play games and knock jumps down with their nose or leg if they feel like it. Not in the ring, just in practice! I jump mostly at Pinto shows where I know that the jumps they use don't go higher than 30" so I don't jump higher than I need to. I set up 4 jumps in the back yard, which I change frequently by adding flowers, latice, fake bricks, etc. and I try to go over the course twice at a time, several times a week. Part of this training is to condition ME so I can make it through the course in the ring! I vary the direction, the pattern, and the heights so they get used to judging distances and how high to jump and try to make sure they can jump CONSISTENT. If they knock a jump down, they must go over it again, even if it has to be lowered so they can do it. After they complete the course (twice) they get to eat a bit of grass. I know lots of people who wouldn't do that but it works for me. That is the only training treat they ever get. I enter both hunter and jumper by training them to trot for hunter, and moving faster for jumper and not giving the command to trot.
Of course, this is after I start them on ground poles, the log and small jumps. Also, I never jump if the grass is wet and they might slip, or of couse, if the ground is icy or frozen.
Good luck! There are lots of photos of jumping on our website.