First of all, you’ll need something like Photoshop. GIMP will do, but these instructions are for Photoshop. Next you’ll need VTF Edit, which you can get from the bottom of the
VTF Edit website. It’s a lot cheaper than Photoshop…
* Now open Photoshop. Create a blank 256×256 RGB 8-bit image.
* Fill this image with mid-grey R 128, G 128, B 128.
* Create a new layer on top of this, and draw / add whatever you want to it. Delete from this layer any bits you want to be transparent in the final spray image
* Press CTRL and click the layer you’re drawing on in the layers pallete, this selects only what you’ve drawn onto the layer
* On the Channels tab, click the second button from the left to “Create alpha channel from selection”
* Go back to the layers tab. Click Layer->Flatten Image
* Now save the image as a TGA file, make sure Alpha Channels is ticked, hit OK and select “32 bit” if a little dialog pops up
* Open VTF Edit, go to File->Import and select your TGA file
* In the dialog that appears, uncheck “Generate Mipmaps” and leave everything else default. Alpha Format should read ‘DXT5′. Hit OK.
* To confirm that yout TGA alpha map worked OK, you can check View->Mask, which should turn on the alpha map preview
* Go to File->Save As and save the file