A pilot hole is either small hole drilled into a material to guide a larger drill to the appropriate location and ease the job of the larger drill, and also allows for the insertion of another hole making tool, such as a knockout punch, that will produce the final size hole, or, in wood or plastic, it serves to locate, guide, and provide clearance for a self-threading screw to prevent damaging the material or breaking the screw. Basically it prevents the screw from splitting the wood and it allows for easier installation in dense hardwoods making sure that the screw is sitting straight. Sounds like a pretty important hole.
Before drilling a pilot hole, you have to determine the precise location of your desired hole. For doing that you can use a tape measure or a ruler. Once you decided on location, use a pencil or a marker to mark the spot.
Preparing the terrain
Before you drill a pilot hole, you need to make sure that the drill bit travels straight down the pathway and does not run off. To do that you make a small indention in the material. Use a nail, a drill bit or some other small, slim tool with a point on one end, and punch it lightly with a hammer.
Proper drill bit
Determine the appropriate size bit for drilling your pilot hole. As a general rule of thumb, your pilot hole should be slightly smaller than the diameter of your screw. If the hole is the same size or larger than the screw, it will not hold. A smaller hole allows the screw threads to grip the material and hold fast. In the picture below you are given the recommendations for pilot hole diameters based on screw sizes.
Drilling the pilot hole
After choosing a bit and fitting it into your drill align the tip of the bit with the indention you made with the center punch. Adjust the drill’s position until you achieve the desired angle of the screw. Drill the hole deep enough to accommodate the length of the screw and maintain a firm grip on the drill in order to prevent it from wobbling and wallowing out the inside of the hole, which will not be good and will increase its diameter. But if you are unlucky and this happens, you will have to use a larger screw for your hole. When you are done, back the bit out carefully.
One thing I have to mention is safety. You should handle your drill carefully and wear protective glasses or goggles so that your eyes are protected from possible attack of the scrapes.