It helps to use humbuck type pickups and the gain on your amp turned up...if the mids are scouped, it's a little more difficult for newbs. Once you get the hang of it, it's a piece of cake...even doing it on acoustic is easy once you've got the feel for it. Definetly not one of the top ten hardest "tricks" to learn.
Its a single motion, get a good bite into the string with your pick and at nearly the same instant the pick pulls of the string the side of your thumb just barely touches the sting for just an instant...hold the thumb there too long and you've got a dead note.
Check this site out...
http://www.cyberfret.com/techniques/harmonics/pinch/page2.php
There's a couple good videos that demonstrate how they're done.
Merry Christmas!