One piece of advice I would share is to know the horse. And when you try things, do it slowly. When you teach your horse to ground drive, that should be something easier for you if you have trained riding horses before, if not, I would get someone who has to help. The horse should understand walk, trot, whoa, turn, and back before you even consider adding the cart. Ground drive them everywhere you plan to drive. Desensitize them to anything and everything you think you might encounter when driving because things are even scarier when you have a cart attached and your vision is compromised by blinders. The worst thing is when a horse gets spooked in harness, and then panics because they feel "trapped". This happened to me recently. The horse bucked and got its back leg over the shaft. Luckily I was quick and got the leg back over, the horse was fine but he bent the shaft really bad. When I was a teenager this happened to me only with a front leg. Not sharing this to scare you, just to make it clear that things can happen, even to the most bomb proof horses.
You can also have someone walk beside you pulling the cart while you ground drive. The horse will hear the wheels of the cart and see it moving beside it and hear it moving behind it. Then you can have the person walk the cart right behind the horse, and even with the shafts part over the horse. That way the horse feels it but if something happens they are not trapped. Slow steps. If the horse is scared at even the sight of the cart, try putting the cart somewhere where the horse can see and sniff it all day. So many ideas for so many different situations. Read some books and watch some videos, and get some live help if you can.