I started my horse ground driving a few months ago. he knows how to lunge and lunges in a surcingle and side reins, he mostly goes off voice command but when i lunge i have a lunge whip just to encourage him along. So i hooked up two lines, one to either side of his bridle and looped them through the dee rings on the surcingle.
The outside line i put over his back just behind the surcingle and i lunged him round as normal. when he got used to this i let the outside line drape over his rump and gradually down behind his hind leg, still lunging him in a circle like normal.
After he was used to this in both directions. I straightened him out and let him walk froward. I kept him at a walk and did little turns, like the corners of the arena. If i felt he was getting a little too fast i put him back on the circle until i felt a bit more in control again. Then let him walk straight again.
It took a lot of work on his part and mine to get used to the tow lines. At first he was a star, i kept dropping one line because i wasn't used to holding it and he'd stop and turn to look at me like "get with the program mam!".
Now we're working on refining our turning aids. i set up cones and have him walk through the cones, and use poles and have him walk both over and through the poles. We started to add trotting but i find he gets too fast for me to keep up with and if i jog along behind him he sometimes takes this as me signalling a want to play and gets a little rambunctious! I've found that because of the extra line in my hands i'm not co-ordinated enough to carry a whip so i just don't, if he turns in to me i just give him a walk on command and sometimes turn his head to face away from me.
he's completely fine with the lines dragging and someone at my barn suggested i get a tire and see if he would drag that along too. i might, maybe a piece of wood or something. i know they want to be able to drive him at some point. the barn manager really wants to be able to get him pulling a sleigh.