How does one modify the moon? (I'm new at this. Can you tell?)

[Calimarfornian]

I've started playing around with the authoring tools so I can make my own mods for the L4D2 workshop. I have experience with 3D model design already, but for my first project, I wanted to make a texture mod for the moon. Simple, right?
Trouble is, I can't figure out what file I'm supposed to edit. I've tried searching for tutorials, but all I can find is tutorials for level mods and flashlight mods.

[Bombraker (Midas)]

I guess that is a texture thing, and you should check the skybox textures :)