I can't help you with the bodykit building. I've got two putty bombs and another kit with all the body panels sawed off. Not pretty sights. Anyway, while I don't believe there is a How-To thread for it, there are two In-Progress threads started by Ratdat that are very detailed, step-by-step process of his body modifications.
http://www.automotiveforums.com/vbul...d.php?t=166816
http://www.automotiveforums.com/vbul...d.php?t=150671
These should get you started. Using styrene for wide body, front bumper, etc. And putty to smooth out lines, etc. Then just go for it. Take a scrap body and try stuff out. You should get the hang of it after some practice.