the basics on training a mini to drive

to do it properly and safely you need some in person help, IMO, but there are some very excellent books. Perhaps your library could get some for you. Basically take lots more time than you think you need. Driving is much more dangerous than riding.

To keep yourself and your pony safe, l would look for a professional Trainer to do it. It takes quite awhile, to make a good cart horse/pony, where they wo not spook or bolt or fight the harness :-) Good luck, l think minis pulling a wagon is so precious!