I think I've exhausted the web looking for a lesson to teach me how to scratch the guitar string with the pick (not that there is anything much about it), but I need to know if there are some rules, for example, scratch from this to this fret to transition between this and that...or scratch (or just make a quick slide up and down the neck) in this or that situation...

Rock and metal players do it all the time, but I wish I could learn more...

Thanks for any help!
I don't understand why you need a lesson on this.

There is no trick to it (or if there is I'm not aware of it) so Chaosinborn put it best.
Your friendship is a fog, that disappears when the wind re-directs (YEAH!)
dude it's not hard at all just do it and use your ears! if it sounds like **** you're not doing it right, if it sounds good you do it right... remember different songs and licks require different picksliding techniques
Thanks guys. You are right, I didn't know it is called "picksliding".

Now that I know, I looked at some tutorials on the web, but still---all of them just tell you how to create the sound by sliding the pick. None tell me what to do with the fretting hand. For example, if I want to hit some chord in the middle of the neck after the slide, do I start the slide and simultaneously slide with fretting hand from 1st fret towards the middle...and that kind of stuff.. I simply cannot find any useful info on the Internet that can actually teach me something..