search for bj's skinning tutorial or I can try to explain here
u need IMGTool,a graphic editing program (I use Photofiltre) and TXD Workshop
open IMGTool then click file>open> go to to your VC directory and double click "GTA3 IMG"
press F2 to search fill in "pcj600.txd" then right mouse click "extract" to any folder you like
now open your editing program and click open then search for the "PCJ600.txd"
edit it to whatever you like and then save it anywhere
now open TXD workshop and open your pcj600.txd
delete the old parts and add the new one(s)
now save it
open IMGTool again
replace the pcj600.txds then click "rebuild archive" and wait untill it's done:D
if there was something you couldn't understand then it was cuz I suck normally at explaining and I was a little confused at thinking=P