Summarize the following:
You'll need to loosen the string that you want to set to relieve tension on the string as you work on other steps. You don't have to remove the strings completely but loosen them enough so that you have room to work under them on the neck. The guitar nut is on the top of your neck, near the headboard. It's what the strings are pulled over before they are wrapped around the tuning pegs. Find a screw that's about the same height as the guitar nut and one that can fit snugly under the strings of your guitar. You may have to search around at a hardware store until you find the right sized screw. Cut off a piece of the screw by bending it back and forth with needle nose pliers. You need the piece of the screw to be small enough so that it can fit under a particular string but not affect the rest of the strings on your neck. This piece of metal will affect the strings intonation. The higher that you slide the screw towards the nut the higher your pitch will be. The closer that you slide the screw to the fret, towards the bridge, the lower or flatter the note will be. Continue to adjust the metal until your note is in tune.

Summary:
Loosen the string that you need to adjust. Find a screw that's the same height as the guitar nut. Break the screw into a quarter inch (0.63 cm) piece. Slide the screw under the string between the first fret and the nut.