While I agree it is definitely in your best interest to take flat lessons, here is the basic run down on how a half-halt is performed. Well, this is how I do it at least.
When you are doing a half halt, your closing off your horses front door. In a sense, your pulling all that energy back to collect yourself, and your horse, to give yourself a more engaged and controlled canter, trot, etc. Your just containing that energy, taking it from being kinetic energy, into potential energy. So lets say your trotting and you want to perform a half halt. Each time your come down from your post, squeeze with your thighs, tighten your abs and lower back, pushing down, but still forward, and apply a little bit of pressure to your reins. Remember, your riding through your legs, to seat, to hands. In essence, your doing HALF of a halt. Literally. The reason you still push your horse forward when you 'halt' is because your telling your horse, we are not stopping, we're just slowing and controlling ourselves. Same thing goes for the canter. Each stride, close off their front door, slow down your seat my using your lower back and adds, pushing down yet onwards with your thighs, and do half of a halt with your reins. YOU set the pace. The only time your horse should is when you really have no idea when to take off for a jump. Half of the time you think your going too fast into a line for a jump, when in reality, your horse knows best. Most of the time a horse knows exactly where and when to take off and THEY will adjust their speed accordingly. We are the ones who usually mess things up. This is why you do a half halt five or so strides before the jump. Never apply one within three strides, as usually things will get screwy. Always always always try to stick with a constant speed. Slow down way before you approach your jump so that your coming into your line at a constant controlled rate.
When in doubt, push on and hold on for dear life :P