You need to pick the string in such a way as the side of your thumb hits the string just after your pick - there are tutorials on this site and all over the 'net. If you know wsomeone that can do it, seeing it happen might be easier...

Use as much gain as you can and select the bridge pickup. As with everything, there's a knack to it - once you get it it's simple - keep practicing.

Best bet to start are frets 5 - 9 on strings 3 & 4. Adding vibrato and bending make the harmonic louder as well....