Programs:
VTF: http://nemesis.thewavelength.net/index.php?p=41
Paint.Net: http://www.getpaint.net/index.html
GCFSCAPE: http://nemesis.thewavelength.net/index.php?c=169&o=75#c4012
STEP 1: Dowload the mod you want replace your OC with
Step 2: extract The ROOT FOLDER IN GCFSCAPE
Step 3: Open VTF and Export it as a PNG.
Step 4: Reskin it The PNG from the mod
STEP 5: Now go into VTF again and open the skin you made it will become untitled now go to the regular skin location and save as the vtf Example [body.vtf]
Step 6: Go To C:\Program Files (x86)\Steam\steamapps\common\Left 4 Dead 2\bin
and Scroll down to until you see VPK
Step 7: Open the move the folder over the VPK
Step 8A: Put the mod in your Addons folder C:\Program Files (x86)\Steam\steamapps\common\Left 4 Dead 2\left4dead2\addons
Step8B:If you have if different Drive Put in there.
Step 9: Open the game
Step 10: Your done.
(If it doesn't work contact me if your stump on a step.)