Create a new brush
Create a new texture in your favorite image editing software (we use Photoshop, but the free Gimp will do just fine).
The specific texture specs are as follows
- .dds format - this is a MUST. The game only uses .dds textures
- 126.96.36.199. ARGB - so, alpha channel is also a must-have as it is used as opacity mask
- 256x256 px texture size
Import brush into the game
When you have the texture, navigate to the Data\Textures\Decals folder and copy the texture into it.
You'll notice a lot of different textures in the folder. They are named by numbers, but that was our approach. The brush textures are loaded into the game and sorted alphabetically so you can name your texture whatever you want.
A few important notes to remember
Something to keep in mind for the sake of mod-compatibility is that it's good to have your textures named specifically, something that would make it highly unlikely that other modders could name their decal textures the same.
And the other thing is the brush color. Black & white (greyscale) images work the best if you plan on colorizing them in-game. If not, if you want to stamp a decal like a logo or a banner, then you can simply leave it as-is and not worry about it. This does NOT mean you should save the texture in greyscale mode (ARGB format is used!), but that the image itself can be black, white and shades of grey.