I first decompress the entire data package in the program files into its original expanded directory setup. From there, I can view or edit any of the game's simpler resources - sounds, text, textures, etc. For the texture, it's just a matter of digging through system files to find Graham's texture image file. I then edit the texture file in Photoshop and replace the original texture with the modded one. Then, I recompile the decompiled system files an run the game using the modded data package.
It's not too complicated, it's just that the decompressing/recompiling steps take a minute or two, so making even one small change to the game's resources requires a few minute of just sitting and waiting.
Another issue is that the texture image is mapped so that each region wraps
around Graham like a cloth, so in plain 2D it looks a bit distorted. This is problematic because I can only edit the texture image in 2D, so a bit of distortion is inevitable. For editing finer details, this usually yields a bit of trial-and-error (which, as aforementioned, is time-consuming).
To illustrate my 2D v. 3D point, here's Graham's texture.