Well, I trained my pony to drive almost completely by myself, so I'll tell you what I've done. She was already broke to ride, so I started out with using a headstall with blinders and the long reins (no surgincle yet). She was a little nervous with the blinders and only being able to see straight in front of her, so my sister stayed at her head to keep her comfortable while I stood a good distance behind her giving her cues through clucking and gently using the reins. If she got confused or balked because the reins worked a little differently, my sister guided her a little bit so she got the idea of what I was asking. Eventaully my sister left her side, and by that point I just ground drove her...everywhere! Down the street, around my house, the barn, fields, so she was completly comfortable. I made sure she stopped when I said whoa, stood perfectly still, backed up, trotted, we did figure 8's, turned around, crossed the arena, anything I could think of.
Then I introduced the saddle (the straps that rest on her withers and come in front of her chest and go under her tail). We went right back to basics, I lead her around in the saddle first, the "crupper" or the part that goes under her tail, took some getting used to, so have patience and use caution. Then I looped the long reins through the "turrets" (I think that's what their called) and drove her just as we always have been practicing. It was a little harder for me to cue her with the reins since they were higher up on the saddle (I was using the reins to touch her sides, now they were postioned high on her back). Again, lots of patience and repition got her comfortable.
Now I STILL have not hooked her up, I think its a huge step, and the horse has to be completely prepared for it. So, next I put on her regular bridle and western saddle, and then connected a rope to the back of her saddle that held a sled (I trained her in the winter). The cords were completely free of oher legs and she could never get caught in them at any pace. So I mounted and we started walking, the sled slid along on the snow behind her, and she was very alarmed. I just walked her with it until the sound of sliding behind her became very normal, and she was fine with it after a week of steady training. Then, one day I put a 50lb bag of grain on the sled, mounted and started walking. She was more surprised at the weight than alarmed, but didn't react to it much at all, we even trotted with it the same day. Next, I had my sister sit on the sled, and she was still very quiet and didn't react to the different sounds and weight.
Okay, FINALLY! We're ready to hook up. A farmer down the rode had a little jog cart, so together we hook her up very carefully and I led her around with the cart for about ten minutes. She was a little nervous, but didn't kick or dramatically startle. Its so important for your horse to be desensitized to an object following her from behind, all the noises, how it feels. Then, I got in, and we drove around the yard like she had been driving for years. To this day I've never had a single problem with her running away or flipping the cart, she's just been a dream. The hardest part by far is getting your horse used to having something following it and having poles/rope touch her sides while she is moving. The sled worked perfectly because it was noisey, could slide very easily without any weight, and I wasn't in danger of her injuring herself by kicking it (it's a flat piece of plastic). Just take care that you arange the ropes high enough so it doesn't interfere with her legs. For my first time training a horse to drive, I know Apache (she's in my avatar!) turned out very well. Good luck! Let me know it this helped any...