From time to time it is needful to screw something into something else, which may not yet accept a screw. If the something else is metal, screw threads can be cut into it using a handy tool called a tap. (For rods, the screw thread cutting tool is called a die).
For a long time this seemed to me like an esoteric and scary task, but when I had to do it, I discovered that it is not difficult at all. It requires a bit of time, a bit of fiddling, and a tap & die set, but is well within anyone's ability who has full use of their hands. I do not even own this tap & die set, I borrowed it from my cousin! You may have a cousin with a tap & die set too. Many people do.
Cutting oil is available at any hardware store. You can if necessary get away without this but I recommend it. We found, having tried it, that lubrication did two things: it made starting the tap a lot easier, and it made the threads a little tighter so that the screw was not quite as loose in the hole. In fact in a couple cases we needed to use a wrench to get the screw all the way tight! This is not a very big deal either way but I suspect that the tighter screw will last longer.
Step 1: What Size?
Screws are labeled by the diameter of the outside of the threads. To make a threaded hole that fits a particular screw size, you need to start with a hole that is slightly smaller: just a bit bigger than the size of the inner core of the screw (as if the threads were all removed). Additionally, threads come in several different gauges, although usually there are only one or two standard options for any given screw diameter.
In our case we had 3/8" screws, and therefore needed to drill holes of 5/16". This information is helpfully printed on the tap itself! The thread gauge for our screw is 16 threads per inch. The tap set had a handy little tool for measuring this, but it is also easily possible to count threads next to a ruler.
Step 2: Preparation
The tap & die set should have two handles, one to hold taps (for inside threading) and one to hold dies (for outside threading). Select the tap handle and unscrew it until the tap you've chosen will fit in, then tighten it again.
Clamp your metal solidly to something that won't move, or otherwise fix it in place. Run a light coating of oil over the edges of the tap screws. You're ready to go.
Step 3: Twist
Insert the end of the tap into your hole. Keeping the tap as perpendicular to the face of the metal as possible, and exerting some downward pressure, start turning the tap slowly clockwise. Very soon you will feel the teeth of the tap start to cut into the metal. For a short while the turning will be wobbly but as the tap progresses downward into the hole, it will stabilize.
I found that a good starting technique was to hold the handle close to my body, and turn with my hips, keeping my hands in the same relation to my body the whole time. I kept my eye on the end of the tap where it was held in the handle, and concentrated on that not moving sideways. After the tap has set it was fine to ease off on this and turn by hand.
The tap works by scraping a spiral groove on the inside of the hole. The metal displaced from the groove in the hole is pulled into the grooves on the tap. You're doing this by hand, and it gets hard to cut the metal pretty quickly -- after about a quarter turn the tap gets very stiff. This is because the piece of metal already scraped out is too big now. To cut it off so it can drop out of the way, reverse the tap direction for a quarter or even a half turn. You will hear and feel the other side of the tap screw edge knocking off the metal scraping.
Keep doing this until the tap is all the way through, and turns freely. The bottom 1/4" or so of the tap, called the chamfer, should be entirely sticking out. Then simply unscrew the tap out of the hole again. Don't forget to clean off the oil and metal shavings!
Step 4: Screw
Wipe off the metal bits and test it out! You should be able to screw your screw in now, possibly with a bit of difficulty if your tapping was uneven at all. I advise one practice hole if that's feasible, for your very first time; it's an easy enough process but there is no substitute for actually doing it to make the explanation all fall into place.