If you've got the patience to read a very detailed book you can find every step of carriage training from green baby to finished pairs horse in a book called "Carriage Driving: A Logical Approach through Dressage," by Heike Bean. If you aren't comfortable with the harness then you should definitely get some lessons from an experienced trainer and learn what the driver needs to know before trying to teach your horse. Meanwhile there's lots you can do to prepare your girl to be a driving professional. Long-line her over, through and around anything you can think of from tarps to cone serpentines to arches full of hanging streamers. Get her used to things making noise out of her range of vision by hanging pots/pans from your belt (seriously!) while you lunge and ground-drive her. Teach her to walk around calmly with two sacks full of tin cans slung over her back. Use a PVC pole along her side to get her used to the idea of shafts (have a helper hold it in the tug at first, don't tie it to the horse until she's comfortable with it.) Work on getting her transitions smooth and calm and teach her to stand calmly for five minutes or more at a time. Don't skimp on that!
Anything you can think of to safely bomb-proof your mare is worth doing. She should be comfortable with straps all around her body and even between her hind legs to prepare her for eventually stepping over a trace by accident some day, and calm about barking dogs, running horses and heavy traffic. If she knows all those things and all of her voice commands teaching her to pull the actual cart will be a snap.
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