If you are using Photoshop for this, or any other program that makes use of layers:Trackmaniack wrote: 4) Copy the DetailsDiffuse.dds file as a new layer onto the DetailsIllum.dds file.
5) Cut out whatever changes you made from the diffuse file that you'd like to make glow in the illum file. In this case, that rectangle.
6) Go to your illum layer and paste it in. I do it this way so that it lines up exactly with the diffuse file. The two are copies of each other, and so if they don't line up, your glows won't, either. (If anybody knows a quicker way of doing this, lemme know.
When you make your rectangle, create it as a seperate layer. Open your DetailsIllum, go back to your DetailsDiffuse and just "duplicate" the rectangle layer to the DetailsIllum. This will duplicate the layer to the exact same location. No fiddling needed afterwards.
Here's my personal garage:
Image
Schweet! 
