I have broke many horses to cart, you are better having some one that knows how to with you when doing this sort of thing. It takes two people any ways, maybe you could see if any of your mates have done it before?

Just to let you know though, I start with just regular ground driving, then do the same in full harness. After I am sure they are feeling good about the whole thing I move up to pulling tires, also if you can get you hands on some pvc pipes, they work great as fake shafts. Better if your horse freaks and starts kicking.

When they are ready to be put to them make sure the first time you don't actually hook the cart on, just put the shafts in the tugs and lead the horse, in case they freak out again that way your helper can just hold the back of the cart and the horse will come away from it.

Good luck let us know what you decide to do and how your pony takes to it.