For starters, as someone who brought her horse to his first dressage show last year, I can offer you some advice that I wish I had. Get a few friends to set up shop at A or C so your horse can get used to coming to a halt with people sitting directly in his path of travel. My horse got a little wigged out about the judges sitting up there at the end of the line and took a good 30 seconds of dancing around before finally coming to a complete stop. We scord a 3 on it with the comment "naughty".
Anyway to practice the actual halt, I would start at the walk on a loose rein. Pick up a nice forward walk and make sure to follow his motion with your seat. To ask for a halt, sit tall, sink deep into the saddle with your seat, and stop following his motion. If your horse doesn't understand what you want, use the reins to complete the halt. Repeat this until your horse begins to halt without needing the reins as backup. If he stops when he feels your position change, that's a sign that he's tuned into your seat.
Now pick up the reins and repeat the excersise with correct contact. You've already got the cues set up, so all you have to do is shut the door with the reins. Think if the reins as just a very last resort finishing step to halting. Be very careful not to pull back on the reins. If anything, bringing the reins a hair forward while asking for the halt will help your horse walk up into the halt properly instead of just bracing against them and jamming to a stop on his forehand.
The last step is adding in the trot since you have to do this move at a working trot. Unlike other dressage test moves where repetition can be bad because it causes the horse to anticipate, I find it beneficial where a good square halt is concerned. Just pick up a good working trot, turn up the center line, transition softly to the halt first with a few steps of sitting trot, half halt your horse to balance him and bring him down to a walk and finally halt. Obviously it has to be done more quickly than that, but once your horse is properly tuned into your seat cues that you're going to stop now, your halts will improve and become more balanced with each one. Good luck.
You just have to see your distance...you don't have to like it.