Instructions
Download new "Left 4 Dead 2" character models from L4D2 content sites, such as Fritzmods, GAmeBanana or Left4deadforums.
Double-click the download file to unpack its contents.
Open your "Left 4 Dead 2" installation directory, which is on your main hard drive in the "Program Files" folder, and open the folder "add-ons." If there is not a folder named "add-ons" in the directory, create one with that exact name.
Place the VPK file from the model download into the "add-ons" folder. Double-click the VPK to install the model.
Load "Left 4 Dead 2" to select your new character model.