How to: tacking a horse

1. Cleaning the horse. Before putting the saddle on the horse, brush any dirt off the horse's coat, which could otherwise cause the horse discomfort if trapped under the saddle (or under other riding equipment). Brush the horse's coat, focusing on where the saddle will sit and including where the girth strap will go.
2. Checking the hooves. It is crucial to check for rocks that could be lodged in the horse's hooves before riding. You don't necessarily have to clean out all the first, especially if conditions are muddy, as the hooves will get muddy again a minute into the ride. To clean the hooves, pick up the horse's foot. Hopefully, the horse will have been trained to pick up its feet easily. Clean out any rocks found in the hoof and check to make sure the hoof is healthy. Set the hoof down and move on to the next one. Be wary of the back feet as some horses are prone to kicking.
3. Fly spraying. In the summer, fly spray is a good thing to have around. Some horses can get very irritated by flies while others don't seem to notice at all. Fly spray can be bought at stores or made at home from recipes.  After the horse is clean, spray the entire torso of the horse from a foot or two away. Spray the legs thoroughly as well and carefully spray the face and ears, being sure to cover they eyes with your hand. For horses that are more head-shy, spray some fly spray onto a hand and rub it on the face and ears.
4. Putting on the saddle. Before putting on the saddle blanket, brush it off in order to make sure nothing is stuck to it (that could otherwise cause the horse discomfort during riding). Set up the blanket on the horse's withers (i.e. the shoulders of the horse - this is around where the mane ends). Next, grab the saddle, making sure the girth strap and right stirrup won't get under the saddle, and set it on the horse's back. Do not toss or drop the saddle onto the horse's back - some horses will be spooked by this. Be sure the saddle is also straight on the horse's back and that no straps are twisted, which could cause discomfort for the horse. Pull the girth strap under the horse's belly and thread the leather through the buckles. Most horses hold their breath when the girth is being tied, so the girth will require adjustment later on. 
5. Putting on the bridle. Start by throwing the reins over the horse's head, then hold the top of the bridle in the right hand and the bit in the left hand. Rest the right hand up near the ears so the bridle is straight and not twisted. Take the bit hand and, while holding the bit, place your thumb and forefinger of the same hand in the corner of the horse's mouth where there are no teeth (and so he won't be able to bite). The horse should respond by opening his mouth and accepting the bit. If not, increase the pressure on the horse's bit until he opens his mouth. If the bridle also has a curb chain, make sure that does not go into the horse's mouth but rather under his chin. With the other hand, pull the top of the bridle over your horse's ears. Buckle the strap under the horse's chin. It does not particularly matter how tight the throat lash is, as long as it is not tight enough to cause discomfort to your horse. 
6. Check all the tack. Now that both the saddle and bridle are on the horse, it is time to check all the tack. Most horses will hold their breath when the saddle girth is being tightened, so you will need to tighten and adjust it again later on. If necessary, you will need to walk your horse around (perhaps over to the mounting block), and then tighten the girth as necessary. Check that the stirrups are the same length, but adjust them once mounted.