Right then. I assume you already know how to do touch and natural harmonics? If not visit the lesson on this site.
To do a pinch harmonic find the strongest point of natural harmonic on your guitar (above the pick-ups not on the fret board).
When you have done that, place your thumb at the top of the string and your pick at the bottom, with only a bit sticking out.
Then give it a twist downwards.
It'll take some practice to get it right. It should squeeelll!