this is a bit hard to describe... u can't learn it totally from text so u will have to practice to get it right... place ur hand as close to the bridge as possible(think palm-muting) then pick a string(preferably d or g string)... bring the outside edge of the pick hand's thumb into light contact with the string on the follow-through of the attack... u can only do a pinch harmonic if u pick the string at a certain area so location is very important... once u get it right try practicing fretted pinch harmonics with bends and vibrato... dun get frustrated if u can't get it right coz it took me about half a year before i could do it
if u find tat u need motivation along the way u can try listening to zakk wylde or ac/dc... their solos are famous for their pinch harmonics...