Getting the original files

in this tutorial i will show you how you can get the original files from the game. 
you need this knowledge to create a mod at all. 

You only need GCFScape.

1. download and install GCFScape.

2. go to the L4D2 folder in steam:
Steam\SteamApps\common\left 4 dead 2\left4dead2

3. open up Pak01_dir.vpk with GCFScape.  
it should look like this: 

4. find the thing you want to modify.

  Weapon textures:    root:/materials/models/v_model
  Melee textures:     root:/materials/models/weapons/melee  
  Survivor textures:  root:/materials/models/survivors
  SI textures:        root:/materials/models/infected
  CI textures:        root:/materials/models/infected/common/l4d2
  Hud/interface:      root:/materials/vgui...
  Scripts:            root:/scripts  

5. extract the thing you want to modify.  

and you are done.

now you can modify the thing.