A horse shoe is meant to protect a domestic horse's natural hoof from cracking or bruising. Becoming a farrier (one who shoes horses) is a skill that requires a clinic on the subject, or it would require some on-the-job training or apprenticeship with an experienced and knowledgeable farrier. A horse's hoof is made of keratin and is much like a human's fingernail--just thicker and tougher. Shaping the hoof and nailing on a shoe correctly can be learned with careful training and observation. Special metal shoeing tools are necessary. Trimming and shoeing a horse is literally a back-breaking task if done consistently, and a poor farrier can damage a horse's feet and balance. As for the horse's comfort, having new shoes put on is a relatively relaxing event for the experienced horse. There is no sedation involved, yet once the horse realizes he's stuck standing there until his feet are done, he may doze as he's shod.
Begin by having someone hold the horse. A very mellow horse may be tied while shoeing, but some horses tend to shift around and require a steady hand by the owner or handler. Most horses relax with some gentle petting on the neck by the handler. First remove the existing shoe on the hoof by using metal pincers or shoe pullers. When a shoe is removed, trim the overgrown hoof wall to the appropriate length using nippers, which resemble pliers, but they are bigger and sharper. Trim the rough and overgrown underside of the hoof from the frog and sole of the hoof with a hoof knife. This is not painful to the horse. Think of a manicure.
Make sure the coffin bone inside the hoof lines up straight with the two pastern bones. If a farrier fails to trim enough hoof in this area, the pastern and coffin bones will become misaligned, and this can cause the horse to experience soreness or lameness in the legs.
Measure the shoe to the hoof and use a hammer and anvil to bend each shoe to the correct shape. If additional devices, such as taps for shoe studs, are added, this is the time to forge them. If you're cold shoeing, the metal shoe is bent without heating it. If you're hot shoeing, the shoe is placed in a portable forge to heat it before bending. The hot shoe can be used to briefly burn a mark on the hoof underside to see if it's lying evenly on the foot, giving the shoe a guideline for a better fit. A hot shoe should not sit long on a hoof as it can burn painfully into the tender underside.
Cool hot shoes in a bucket of water to lower temperature. Nail the cooled shoes on, by driving nails into the hoof wall along the white line of the hoof. The nails are shaped specially so they bend out as they are nailed in, to avoid hitting the sensitive inside area of the hoof. (A farrier may rarely drive a nail into the softer tissue, causing pain. The horse will react by pulling the foot away. The horse may be tender-footed for a few days if this occurs, but no long-term damage will be done.) Hammer nails until they come out on the exterior side of the hoof. Continue hammering until flush with hoof bottom. Cut off the sharp ends and use special tongs called a clincher (or a clinching block and hammer) to bend the remaining end of the nail so until it is almost level with the exterior hoof wall. This gives the shoe a secure fit.
Use a large file called a rasp to smooth down the outside edge where the hoof meets the shoe and file down any sharp edges where the nails were cut off, leaving a smooth lower hoof. The whole process should take less than an hour, and it is painless for the horse.