I support everyone else in gettting an instructor and being safe. The instructor does't need to be a mini horse trainer - just willing to start you out with your lessons at first and then to help you with your mini in the future.
There are several books and some videos out as well that are good for beginners. The two "tried & true" ones - Doris Ganton's Training the Driving Horse (about $18 on Amazon?) and Carriage Driving by Heike Bean - also on Amazon or thru the American Driving Association. Many Mini horse tack places also sell these two books.
Videos - especially on a budget and can only get a couple - Training the Driving Horse by Mary Ruth Marks. Otherwise, again, there are now several that are out there by/for mini horses - offered on mini horse tack sites.
Rural Heritage also puts out quite a number of books on driving. While many are meant for Draft Horses - they have a LOT of info about different harness styles, fit, and use; surprisingly (or maybe not?) - there are several which give GOOD explanations of how to hold lines/reins for driving and how each "hold" or way of handling the lines works, with lots of pictures as well ("Training Horses, Training Teamsters"; "Farming with Horses"; "The Work Horse Handbook").
Then there is this forum.
Leia had a set of diagrams that she puts out - describing the parts of the harness and how each part was a portion of a "system" and how each system worked. Direction (bridle and lines), pulling (breast collar & traces), brakes (breaching, hipstrap or backstrap assembly & crupper, holdback straps) & carrying (?) - (saddle, shaft carriers, wrap straps).
Also - search for "Training" (
http://www.miniaturehorsetalk.com/index.php?showtopic=125628&hl=
This is a training post that many of us were posting on... Has plenty of great pics! and personal descriptions of what quite a number of us have done while training our horses and ponies - including possible mistakes that one might be able to avoid.
Last but far from least - each of us develops our own way of harnessing our horses. There are several ways to do so - safely. Generally, especially in public (show or event), you never harness your horse unless your horse is tied or held by a helper/groom.
For me - while doing our original training - the bridle may already be in place while grooming my horse. I start w/o blinders as I want my horse to see and accept everything that we do with harnessing, ground driving and first hitches. I usually make my own headstalls - with snaps for ez on/off. AFter horse is started, I go back and do everything again with the blinders in place.
When I harness, my horse may be tied up or he may be "loose" (the lead line draped over my forearm where I can grab it). If headstall not already on, I usually put it on last. UNLESS I'm at a show or working with a fully trained driving horse, I almost always leave a halter on under the bridle. I DO put the bridle AND lines on the horse before he is hitched - whether ground driving with a tire or implement or hitching to a cart. For breast collar type harness - I put the saddle on and do up the girth. I then buckle the wrap strap into the shaft carrier to keep it from being stepped on (that would be the side where the girth is being done up -it's already buckled in on the other side). I then add the back strap, crupper and breeching. Do the hold back straps up to keep them from dragging as well (if they aren't already buckled up out of the way). I put the breast collar and traces next - drawing the traces thru the breeching or thru the hip strap assembly to hold them up off the ground until ready to hitch. Last, if headstall not on, I put it on. Then attach the driving lines, IF the horse is ready to be ground driven or hitched.
And I try to train my horses to accept complete harnessing from both sides - most of mine at one point or another are going to drive as a pair or in larger multiples. It's easier to harness two animals next to each other by harnessing the left one from his near side (left) and the right one from his far side (right).
I follow many other steps before I attach the lines directly to the bit, while training.