Excerpted from The Alex de Grassi Fingerstyle Guitar Method
Natural harmonics are produced at specific locations, or nodes, that divide an open string into equal parts. The pitch is determined by the number of divisions. Dividing the string in half at the 12th fret produces the first harmonic; dividing the string into three parts at the seventh fret produces the second harmonic; dividing the string into four parts at the fifth fret produces the third harmonic; and so on. The pitches and tab for all the natural harmonics at each of the first four harmonic positions are notated in Example 4a. Note that because some of the pitches are very high, the symbol 8va is used to indicate notes that sound one octave higher than they are written.
Place either the third or fourth fretting-hand finger so it rests lightly across all six strings at the 12th fret. (You can use any finger, but it’s good to get in the habit of using the third and fourth fingers, especially in the higher positions because those are the fingers often used in practice.) Rest the pad of the finger lightly on the sixth string without pressing it down. Pluck the string with the picking-hand thumb and then immediately lift the fretting-hand finger off the string. This should produce a bell-like sound. If the harmonic does not sound, try using either more or less pressure with the fretting-hand finger, and experiment with how quickly you lift the fretting-hand finger off the string after plucking it. Be sure the fretting-hand finger is directly over the fret.