This is a quick tutorial covering the basic steps involved in creating custom weapon skins for CS:GO. The skins created can be submitted to the Steam Workshop for a chance to gain votes and be added in the game. However, unless they are officially added, your skins can't actually be used in game except in private offline matches.
To make skins you will need either Photoshop CS6 extended or GIMP, VtfEdit, and notepad.
Step 1: Getting the Files
To get started, you first need the weapon files which can be found on the Steam Workshop page. In the folder are each weapon's .obj and .tga files. The obj is the 3D model and the tga provides the UV sheets that make up the actual texture of the weapon model.
Step 2: Starting a Texture
All actual texture mapping is done in the tga file. This can be reached either through the "edit texture" function in the obj, which shows and updates the texture on the model in real time, or by opening the tga directly. The file will show a UV sheet like this one for the AWP. They can be a bit hard to read, so it is best to work with the updating obj. Each piece of the weapon is outlined in green, and each plane of the weapon is shown in white.
Step 3: Create Your Texture
From here, the tga file is like a coloring book. Fill each section with the desired color or pattern. First putting a base color lay is highly recommended. Once your design is complete, save the file as a tga in a place you can easily find it.
Step 4: VTF and Notepad
After the tga file is complete and saved, import it to VtfEdit. Then export the image to save it as a vtf file. The last file needed for the workshop is a text file leading to the location of the vtf. The format for the text file can be found on the Steam Workshop page, all that needs to be done is entering the path to your vtf file in the first line.
Step 5: Finish in the Workbench
The last steps are done in the CS:GO workbench. Upload the file, see how it looks in game. Save it to the workbench and hit "submit." Then just load the files in the given spaces and you're done!