Simplest and shortest: play any string at any fret like you normally do, and after you pick a note lightly touch the same string you plucked with your right hand finger 12, 7 or 5 frets apart from the original note and you will get a tap harmonic
tap on the fret wire 12 frets above the note you're fretting.

tap harmonic.
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i find its not the best way to do harmonics. sure it looks cool but pinch harmonics are much easier and are usually louder.

to get the octave, gently and carefully tap the octave above the note you fretted. try not to make contact with the fret when you tap. you can also tap fifths. havent done any more than that though.