Has your horse ever been long lined when in training? It's basically the same thing. My family had draft horses and hackney ponies when I was at home and now, we have a haflingers and I have 2 paints.
Does your horse lunge well in both directions? Do you use side lines when you round pen him? That's the start. There is a book, it's an older one, that has the basics for driving - Breaking and Training the Driving Horse by Doris Ganton. Published by Wilshire Book Company. As I said, it's an older book yet it covers the basics.
Once you get gound driving, you'll have to go all the way and get a cart - driving is awesome.
Make every day your best day yet.