It doesn't have to be steam, as long as it has a .gcf library and uses the source engine
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Step 1: You need programs.
These are your saviors in that matter:
A program used to open GCFs "Game Cache Files", which are actually folders containing all the games files.
Get it here:
While browsing the gfc, this program is used to open VTFs "Valve Texture File" These are the materials and sprites used in your game of choice.
Get it here:
Step 2: Working GCFscape
A new window pops up saying "open package". Here you browse your way to
note: where i wrote "program", on you computer it might say "programs" or "program files". On Windows 7 64 bit, it's "Program Files (x86)"
When you're there, you'll find several files with blue cubes as icons. These are the GCFs.
Now, if you want to edit portal files, double click "portal content" (if you have portal,that is. It will work with any other game, yet portal will be my example)
At this point you will find folders. Click on the folder with the game's name, and then the materials folder.
Browse around until you find a VTF (not vmt) file that seems interesting, and open it with VTFedit.
Memorize the path!
Such as: Portal/Portal/Materials/Models/Props
Step 3: Working VTFedit
It merely turns your materials into regular image files. Then you can go on an editing rampage!
converting a .vtf into .jpg or whatever does not remove the vtf, but creates an image file with the same name!
Step 4: The editing
I get to edit it!
Open the converted image and do your thing!
I use mspaint, but photoshop is probably better.