For the safety of you and your horse you need some hands on experience with an experienced driver. We joined our local carriage club and the help is great. Driving is a lot more dangerous than riding and you kind of need to start over with your horse. Not putting the horse before your cart means doing your ground work first. Teach your horse to ground tie (stand unattended) so that it will stand for harnessing without moving. Ground drive your horse with a surcingle and long lines until it is consistent in its turns and stopping. I longe my horses to teach gait transitions: walk, trot, canter, slow trot, jog, stop by voice command. You may not want to jog when you are driving but the ability to travel at a slow, controlled
gait is important. The stop should be instantaneous and consistent without taking steps afterward. I have a tow bar that I made out of a piece of two inch pipe with open hooks on it to attach the traces or even rope to. I next pull an inner tube, then a tire and finally a 8 inch by six foot long log around the arena practicing stops and circles. If the horse gets excited and turns around the bar detaches. The first time I hook my two wheeled cart to the horse I lead him it a bit. When I begin to drive the horse outside the cart either from the side or behind the cart I have a helper at the side of the horse's head with a ten foot lead line attached to the halter under the bridal for control if needed. When you first enter the cart a helper should be at the head of the horse. Before you venture out on trails you should have many hours driving in an enclosed arena practicing transitions, circles, figure eights, serpentines etc. This may seem like a lot of work but it's all part of having a safe driving horse. Through each step you should be honestly evaluating your horse and asking yourself if it has the temperament to be a driving horse.