Now that you have the horse, carriage and harness it is time to put it all together. Probably the easiest way would be to enlist the help of a seasoned professional to help you with this. If that is not possible I will explain the basics of the process.
Gather up your horse and prepare him much as you would if you were going to ride him. Brush and clean him off, check his hoofs, make sure all of the goobers are out of his eyes and braid his forelock which will help when putting the bridle on.
Next make sure that your harness is put together. If you purchased a new harness, they generally will come in pieces so you will need to put it together. If you purchased a used harness then it is probably already together, but you should check to be sure that there are no breaks in the leather and that there are no pieces missing.
Now start harnessing the horse. First put the saddle on and loosely buckle the girth. Next put the crupper around the horses tail making sure not to catch any hair from his tail and buckle it. Put the breast collar on by buckling the neck strap over the withers, and lay the traces over the back of the horse so they are not dragging on the ground. Then undo the girth and run it through the false martingale and buckle the girth tight. Finally put the bridle on being sure not to bump the blinkers against the horses eyes. Fasten the throat latch and the caveson. Attach the reins to the bit then run them through the terrets on the neck strap and the saddle, fold and lay across the horses back.
It is now time to put the horse to the carriage. This is when a second person comes in real handy to head the horse while you are hitching. Bring the carriage up from behind putting a shaft on each side of the horse and slipping the end of the shaft through the shaft tugs, or if you have quick release tugs bring them around the shaft and buckle, all the way to the tug stops on the shaft. Next working one side at a time bring the trace back to the carriage an slip the large buttonhole over the single tree ends, or if you have quick releases then connect to them. Attached the breeching strap through the footman loop, wrap, then buckle back to itself catching the trace in the loop that you have made. Then buckle the tug strap to the overgirth strap to hold the tug in place. Repeat this process on the other side of the horse. Lastly bring the reins back and buckle them being sure that they are not twisted, get in and have a safe and pleasant drive.
Remember there are many styles of harness and different makes are built with little differences. Using quick releases to connect your traces, breeching straps and shaft tugs onto your horse is a good idea for in an emergency you are able to get your horse out of the carriage quicker then unbuckling every buckle, because an emergency is just that and the quicker you get unhitched the better. Although quick releases are a great help they to can become stressed to the point that they will not release so always carry a knife on your person. Replacing some harness pieces is a whole lot cheaper than loosing a horse or injuring a person.