I agree with the others about shaft training and desensitization- especially the later. I would spend a lot of time on getting him used to sounds around and behind him, to the point where he doesn’t flinch or tense up when something makes a funny noise behind him. Do this staring out just leading or lunging without blinders so he can see the scary objects, then slowly progress to long lining with blinders with the noises. This can save you a lot of time and money with a trainer if you have the horse broke by someone else, if you already have the horse bomb proof, used to all of the elements of pulling a cart, and long lining through fire, basically.
To teach them about pulling against weight, I don’t like the idea of hooking them to something to drag around- at least not initially. I generally attach a lunge line or two onto the breast plate, then pull on that while long lining- usually requires a second person. Once they’re good with pulling against resistace (which you need to teach them to walk on against it), scary noises behind them, and the feel of shafts at their sides, then I start to combine these elements- preferably noises and weight first, then shafts and weight, then shafts and noises once they’re rock solid with everything else. I don’t actually tie the horse to anything initially, but have an assistant operating these functions themselves. You long line +/- drag the noise maker, and your assistant pull back on the traces (they can also drag the noisy thing if they can do both at once or if you’re not quite as handy with the lines/whip). By doing it this way and not tying things directly to the horse, you can drop the extra elements at any time and the horse can become free of these other stimuli instantly should he spook or need schooling.
Once they’re used to all of those things, then slowly start introducing to the cart without actually hooking the horse to it. This is where I’d enlist the help of a pro if you are green. The pro can work with you and the horse to decide when it’s safe to move on to the next stage of the training.
I personally don’t think it’s a great idea to try to teach a horse to drive if you’re unsure of the steps, but I also think that most good horsepeople who are good at long lining and handy, can do some of the preparatory steps themselves and shouldn’t have to rely on a pro for every step along the way. We all have to start somewhere. Get a good foundation on the horse, then enlist experienced help when you get to the actual cart/hooking horse to objects phase.