Step1
Know standard tuning on a guitar. The lower notes are on the thicker strings, which are on the top half of the guitar when you are fingering the chords with your left hand and strumming with your right (With a lefty guitar, everything is done in reverse.) The open strings, from lowest to highest, are E, A, D, G, B, E. Notice that there are two E's.
Step2
Set the low E string to E. You can do this by getting a note from another guitarist or playing a pitch pipe and copying the tone.
Step3
Touch the low E string lightly above the fifth fret with your left index or middle finger and pluck the string with your right hand or a pick, removing your left hand from the string the moment you do. It should ring out with a high pitched tone: the harmonic. This step requires timing and may take a bit of practice.
Step4
While the E string is ringing, pluck the correct harmonic on the A string, the second lowest string on the instrument. Instead of placing your finger above the fifth fret of the A string, place it above the seventh.
Step5
You will hear a beat frequency between the two strings. A beat frequency is a "wah-wah" sound of a tone rapidly getting louder and quieter. It is produced by the interference between two strings playing similar, but not identical notes. If, for example, one string is vibrating at 400 hertz (or cycles per second) and the other string is at 402 hertz, you will hear the sound get louder and softer two times per second.
Step6
Tune the A string to the low E. You will hear the beat frequency get slower and then stop. Once it stops, you are in tune.
Step7
Tune the D, the next string after the A. Pluck the A string at the fifth fret and the D string at the seventh fret. Adjust the tuning of the G string until the beat frequency stops.
Step8
Tune the next string, the G, in the same way. Play the fifth fret harmonic on the D string and the seventh fret harmonic on the G, then tune to eliminate the beat frequency.
Step9
Tuning the B sting using harmonics is a little different. This time, play a fourth fret harmonic on the D and the fifth fret harmonic on the B. Tune the B to eliminate the beat frequency.
Step10
Tune the high E. Play the fifth fret harmonica on B and the seventh fret harmonic on high E.
Step11
Play the high E and the low E together. If they do not play the same note then your tuning is a bit off, and you will have to go through and retune. You will usually here a beat frequency between them if the tuning is off.
Step12
Play a chord. With a little practice, you will instantly be able to tell whether or not your guitar is in tune by whether the chord sounds "sour." If it doesn't sound right, retune.