hold your pick so that only a few mm of the tip (sharp end) is visible, this means when you play the note your thumb will brush the string causing a harmonic, but only if your pick hand is the correct distance from the note you're fretting, so first fret a note eg; any fret on your g string between 3 and 9 [i find them easiest there , it works most anywhere]
then starting at the bridge and working forward pick the string with a sharp percussive attack, at the same time vibrato the note,
keep trying til you find the correct distance(there's more than one) with practise you should find the distance instinctively.
an easier but more limited way is to sound a natural harmonic over the fret-, pick normally then press down and vibrato (this sounds great but only on wound strings and on the 2nd, 3rd,and 4th frets- good for spicing up those chugging e riffs.
sorry i can't explain it better, but persevere and once you get one you'll be squealing like a pig!
