Instructions
Go to the GameBanana website and click on the link that takes you to the "Left 4 Dead Skins" download page.
Browse through the available skins. When you find a skin you like, record its name and page number so you can continue browsing and easily come back to it.
Click on the skin you want. You are then taken to the specific page for the skin. Click the download button.
Double-click the title for the download in the "Download Menu" and save the files to the default folder: "content,contents,root,drive_c,program files,left 4 dead 2,left4dead2,addons."