Summarize this article:

On the saddle are leather straps called the girth (or the belly band). Feed the leather strap into the buckle, and secure it so the girth is tight. You should be able to fit your fingers underneath the strap. The girth stops the shafts from going backward if the cart or carriage gets unbalanced. Take the leather straps of the traces, and place each of them behind the leather straps of the belly band. They should sit on top of the saddle’s leather strap (behind the horse’s front legs). The bridle will easily fit over their head. Then, place the bit inside the horse’s mouth so their tongue lies underneath. Attach the curb chain to the hook on both sides of the bit so the bridle stays put on their head. The bit is a slightly curved metal piece that sits in the horse’s mouth. This helps control the horse. Connected to the bit is a curb chain and 2 rings for the reins. After the bit and curb chain are in place, strap the reins to both of the rings on the bit. You can use the lower loop for more control over the horse. Run the reins through the rein tariffs on the harness and buckle up the ends.  More control of the horse is helpful if you are driving on roads with traffic, for example. The rein tariffs keep the reins straight and tangle-free. Inspect each buckle and ring to ensure each strap is properly fastened. They should fit snugly around the horse, and you should be able to fit your fingers underneath. Tighten or loosen the straps as needed.

Summary:
Attach the girth around your horse’s body to keep the saddle in place. Run the traces behind the girth. Place the bridle around your horse’s head and the bit in their mouth. Connect the reins to the bridle and the saddle. Check each strap to make sure it fits securely and comfortably.