Ground drive with your harness saddle. Sometimes it helps to put the reins through the tugs instead of the terrets. If your reins aren't long enough, tie some clothesline onto them. The horse doesn't understand, at first, why you are BEHIND him. If you feel he needs a little practice having shafts, you can make some PVC shafts for him to drag. I had mine drag evergreen trees. We have a lot of pesky junipers here, so I'd cut them for my hrose to drag. A good whoa is very important. It is nice if you can have a few lessons from someone, but don't be afraid to try by yourself. You will know your own horse and when he is ready to be in the cart.
I get behind mine when he's tied for grooming and have lessons in moving away from the whip cue. I tap on his side and ask him to move over. I ask him to move back and forth a few times. At first he doesn't understand, but if he's sensible he learns about moving away easily. If he isn't sensible he probably won't be good for driving anyway.
The first time you go out in the cart, it is a good idea to have someone walk along as a header. Some horses have a few lessons in ground driving and take to the cart. Some are not ever comfortable or reliable in the cart. You will know your own horse best.
All the driving books are great, but sometimes the information is too overwhelming and off-putting. Understanding what the parts of the harness do is helpful. You will probably need help getting the harness on, but you will also find that nearly everyone has a different technique. The harness does not come with instructions! You get this delightful-smelling pile of leather in a box. Sometimes the straps are buckled where they would never go, just because it makes shipping easier.
Dapper Dan and I learned on each other. I made mistakes, but we have been having fun together for many years. I've had some lessons since, and they help a lot, but if I had waited for professional help to get started, I might still be waiting! And think of all the fun I'd have missed.