I've been training miniatures horses to drive now for a few years, and I believe every miniature should be trained to drive. Minis need a job, they are smart and need the fulfillment of work. I also agree finding help from someone who knows what to do is wisest, however I know it's possible to teach yourself and your mini as long as you understand horses and their body language. Step one, is lunging in a round pen. You want your mini to know its verbal commands perfectly before you even ground drive. Make sure your mini can walk, trot, canter, stop, and backup on verbal command. This will make ground driving much safer to teach your mini and less confusing. Teach in a controlled environment if possible away from distractions. Also I wanted to tell you about carts. I build four seater carriages so I know a few things about them. When you do get to the point you can hitch, make sure you have watched videos on how to properly balance the cart. Two wheel carts do apply some weight to your minis back and you want to make sure that is no more than 10-15lbs. You also do not want the shafts angled so high that weight is lifting up on your mini's belly. If you watch some videos on youtube about balancing a cart you will understand what im talking about. Two wheel carts are easier to buy, but eventually I suggest you get a carriage with four wheels. Carriages apply little weight to the minis back, because all the weight is on the four wheels instead. I also firmly believe they are safer because if your mini trips a two wheel cart crashes all the weight down onto them. It can be very scary and dangerous. However a carriage holds itself up and can actually help catch your mini from tripping, or at least not crash weight down on them.
If you live where there is sand or snow, I actually think starting out with a dogsled is a really safe way to adjust your mini to pulling something. But the sand has to be wet and your own weight needs to be under 130lbs or it will be too hard for your mini to pull. You can walk behind it (not on it) and place a weight on the sled instead. Use a head collar harness for a sled and not a breast collar harness. My mini loves his dog sled.