It's not magic. It's knowing how and where to hit the shoe to get the response you want from your work. Now I'm going to tell you how to work a shoe cold, because it is the most common method used by (How do I say this without offending anyone?) non-professional horseshoers.
After you trim the hoof (see Hoof Trimming Tools and How to Use Them), but without rounding off the outside edge of the hoof wall, select a factory-made shoe of appropriate size. To check the fit, choose a shoe you think is the right size, pick up the trimmed foot, put the center of the toe of the shoe to the center of the toe at the ground surface of the hoof. Move the shoe from one outside hoof wall to the other and see if the shoe covers the hoof wall. The shoe must be long enough to cover the hoof wall, but not more than 1/4" longer than the hoof wall on the front. A little longer on the rear is all right.
To help develop your eye for hoof shape, trace the outside of the hoof onto a piece of cardboard, or measure the widest part of the hoof and the heels, and make the shoe to those measurements.
Even though the shoe you are shaping is cold, hold it with tongs. The vibration from hitting a shoe with a hammer can be painful to your bare hand. The tongs also serve to keep your thumb and fingers away from the hammer, which sometimes has a mind of its own.
If the shoe is narrower than the hoof at the toe, you will need to open the shoe at the toe. Holding the shoe with the tongs, place the shoe over the horn of the anvil, toe up. Slide the shoe back on the horn to the widest part of the shoe. With the hammer, hit the shoe as squarely as you can in the middle of the toe. As the shoe opens up, move the shoe back on the horn and hit it again. If you need more width than you can get from the horn, move the shoe to the anvil's heel. When you think the shoe has opened up enough, check it for flatness. You can check it by looking across the shoe or by placing the shoe on the face of the anvil to see if it rocks or lies flat.
If the shoe is not flat—let's say one heel sticks up—do not try to hit the heel down. It will not bend and the shoe may take off like a spring. Instead turn the shoe over and look for a bridge in the shoe between the toe and the bent heel. Holding the shoe with your tongs, use your hammer to smack that little bridge flat against the face of the anvil. Always hit the bridge and not the spring, or you will spend a lot of time looking for shoes in all the wrong places.
After again checking the shoe for flatness, take it back to the hoof and see how much you messed it up. If the toe fits well, but the branches are too wide, go back to the anvil. Using the tongs to hold the shoe, put the shoe over the horn of the anvil, pull the shoe toward you so it rests on the horn at about the first or second nail hole in the shoe. Hit the shoe with the hammer below this point. As you hit the shoe lift it on the horn to the next nail hole and hit it again. Keep going until you get to the heel of the shoe, then roll the shoe toward yourself and hit it again and again. Now check the shoe for flatness. If it needs flattening, you know what do.
Work the shoe's other branch the same way. When bending a branch on the horn, keep the tongs directly across the shoe from the spot you want to hit with the hammer.
Take the shoe back to the horse and try the fit. If it fits, nail that shoe to the foot. If it doesn't fit—let's say it's too wide—use your tongs to hold the shoe on its edge, vertical to the anvil's face, and hit the top branch of the shoe with your hammer. This action will narrow up the shoe.
If the shoe is too narrow, widen it by holding the shoe with your tongs over the horn and hitting the toe with your hammer. Another method is to hold the shoe with your tongs vertically over the face of the anvil with one heel on top of the anvil and the other heel over the far edge on the side of the anvil. Hit the toe of the shoe with your hammer. Check the shoe for flatness after making each adjustment.
If the horse's hoof is straight on one side, or to make the toe of the shoe more pointed for a rear foot, move along to the anvil's heel. Holding the shoe with your tongs place the branch of the shoe that needs straightening on the top of the anvil, with the other branch under the heel of the anvil. Place the part of the shoe that needs straightening on the flat and hit the shoe with your hammer.
The number of times you need to hit a shoe with your hammer depends on the size of your hammer, the size of your arm, and the size of your determination.
Any way you look at it, shaping a horseshoe to fit a horse's hoof is not magic. It just takes practice.