Saws are common and useful tools, but to work properly they must be kept sharp. A sharp saw spares a lot of extra effort in working with wood or any other substance. The occasional user might need to sharpen their saw infrequently enough that they can take the tool to the shop for this service. But a regular user will need to learn how to do it themselves.
Clamp the saw blade.
Place the saw file against the front edge of the first saw tooth, starting down by the handle.
Set the saw file snugly between the teeth, holding it at an angle of about 60 or 70 degrees.
Make a stroke square across the tooth with the saw file.
Repeat this 2 or 3 times, or until the saw tooth's blade is shiny up to the tip.
These instructions are for a crosscut saw, which is the most common type. For a ripsaw, simply adjust by running the saw file directly across the teeth at a straight, 90 degree angle instead of the 60 to 70 degree angle.