Installation details in downloadable file or here.
Add the following line into your whitelist.cfg:
"add" "materials/models/..."
Then go into your pak01_dir.vpk file through Notepad++ and press CTRL+H.
Write in the "Find what box":
$baseTexture "models\infected\hulk/Hulk_01"
And in "replace with":
$baseTexture "models\infected\hulk/Hulk_02"
And press "replace".
Now copy the "materials" directory into your "../left4dead/" folder
Enjoy!