You have to tap directly onto the fret (onto the metal part), not in between the frets. To know which fret you need to tap exactly, it works the same as natural harmonics, 12th fret, 7th, 5th etc. the 12th fret is easiest to achieve. So for example If your left hand is holding a note on the 5th fret, you tap onto the 5+12=17th fret.