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.