In TF2, war paints are textures generated on the fly using several stages :

• Texture lookup : stage needed to load the textures and apply some transformations : rotation / translation / scaling + color correction. This is the stage we are interrested in.
  
• Select : select up to 16 parts of a weapon using a special texture. Patterns will be applied only on the selected parts
  
• Combine lerp : Combine 2 textures using a third one as a stencil. Usually the stencil is the result of a select stage
  
• Combine multiply : Multiply the input textures.
  
• Apply sticker : Same as texture lookup, except the transformations are not random

A RNG using the item's original ID as a seed will be used to randomize the scale / rotation / translation / color correction / sticker choice among a range depending on the item's wear.

Therefore the appearence of a weapon is only defined by its original ID and its wear

Select a character and add the desired weapon. Click the war paint icon .
In the appearing window, select the desired warpaint serving as a template and optionally wear / seed. Note that changing any of these options reset the editor. Open the warpaint editor using the shortcut SHIFT+W

Locate the stage "Texture lookup" containing the pattern you want to replace. Click on the picture and select your texture. Note that the VTF file format is not accepted yet, you have to convert it to png if necessary. The warpaint should update automatically.

Repeat this operation for each pattern if desired. Once you are done, locate the rightmost stage containig the final texture and click the "save picture" button.

For the sake of the demonstration, I'm modifying the "Alien tech" warpaint. Alien tech is a 3-patterns texture. Most of the war paints have 2 or 3 texture, more rarely 1.


I'm replacing the "tech" texture (top second from the left) and green texture (bottom middle column) using CSGo textures.