You have created a skin and now you want to see what it looks like in-game.
Put files in the correct places
Navigate to your Garry's Mod Installation directory (e.g C:\Program Files (x86)\Steam\SteamApps\common\GarrysMod)
Go to the "garrysmod" directory and then materials\models\weapons (If any of these directories do not exist, feel free to create them)
Create a directory called "warzone_skintest"
Go in to the "warzone_skintest" folder.
Copy your VTF to this directory. In this guide I am using p90_giftwrap.vtf
Once the file has been copied, right click > New > Text Document
Check folder options!
You need to go in to the Windows Folder Options and untick "Hide extensions for known file types"
Rename the file to <weapon>.vmt - For a list of material names, go to: List of Material Names
In this case, the P90 is being used, on the material list, the one I want is "materials/models/weapons/warzone_skintest/p90.vmt" so I am going to name this folder "p90.vmt"
Next, right click on the newly created VMT and click Edit. You can use any text editor but Notepad++ is recommended.
Depending on the type of item you are creating, the VMT contents is different. See Example VMT files for more information.
In this example, I am going to use the "Weapons" VMT example
Paste this in to the text editor and change the $basetexture value to match your vtf name (Without the .vtf extension)
Save the VMT file and that should be it!
You can now go in-game and test this.
Open Garry's Mod and connect to the Skin Testing Environment
Once connected to the server, open the console and type in the following command:
After that, type !options (Or /options, whatever you prefer)
Select "Content Creator" and then click on which ever type of items you wish to content create for.
Once completed, go over to either the Weapon, Knife or glove dealer and press E (or your default USE key)
You will now see an extra option on the side for Content Creator Weapons. Click on this and you will get a list of all weapons.
Select the skin you wish to test.
If the materials worked, you should see your skin.
You can also access the same option on the other dealers.