I start with lots & lots of pole work. We trot single poles, we trot series of poles set about 9ft. Apart so they can take a whole step inside, then 3-5 ft. Apart(depending on the size of the horses step/stride;3' being a small/medium pony, 5 being closer to a large horse), and then work on cantering single poles, and then a series of canter poles 9' apart. You can shorten the poles to make them learn to collect, and lengthen them to work on lengthening their strides. When the horse has become comfortable with the poles, then you can raise them & make them cavalettis. When the horse is very schooled & confident over the pole & cavaletti exercises, then start with small crossbars. If the horse is rushy to the fence, or they aren't very interested, then I throw trot poles in right before the fence. He could be knocking them over, simply because he doesn't know & he isn't very impressed by the height/filler. Make sure you keep your upper-body up & still & looking forward, and keep your leg closed & supportive before & during the jump. Good luck!