- 1). Tune the guitar to pitch, using the string gauge (string size) you plan to use on the guitar. It's important to have the guitar strung to pitch because strings put pressure on a neck and actually bend it. If the action is set up on a guitar that is not tuned properly, then the action will not be correct once it is tuned. Changing string gauge will also create a different amount of pressure on the neck, which can affect the guitar's action.
- 2). Place a 24-inch steel ruler directly on top of the frets. The ruler should be placed on its edge and should extend from the first fret to at least the 22nd fret. Measure the gap between the steel rule and the 12th fret with a set of feeler gauges. These are metal strips of various thicknesses. The proper relief for an acoustic guitar is between .070 inch and .078 inch.
- 3). Place a five millimeter Allen wrench into the truss rod nut. This is located inside of the sound hole where the neck meets the top of the sound board. If you look inside of the hole on the face of the guitar and look toward the neck, you will see the Allen nut.
- 4). Turn the Allen wrench clockwise if the distance between the 12th fret and the ruler is less then .070 inch. This will create more of a concave bow and reduce the possibility of the string hitting the fret when vibrating. Turn the Allen wrench counterclockwise if the distance is greater then 0.78 inch. This will straighten out the neck, thus decreasing the distance between the strings and the frets, making it easier to press the strings against the frets when playing.
- 5). Measure the distance between the fret and the steel ruler at the 12th fret. If it is not between 0.70 inch and .078 inch, then repeat step four until it is. Once the action is correct, remove the steel rule and tune the guitar to pitch. Changing the action can cause the strings to go out of tune. Once the guitar is tuned to pitch, you should play it. It should be easier to press the strings and there should be very little buzzing.
Source...