Alright I decided to make a quick tutorial on this. My steps are intended for GIMP but I'm sure PS is extremely similar. Really basic tutorial with a sweet end result.
Courtesy/credit to my friend Mercenary from
www.gimptalk.com for showing me this awesome effect.
End result:

Step 1
Take out any sig you would like to use. (Darker sigs usually work better for this because it's harder to see with brighter sigs.)
Step 2
Save this picture here:
http://i123.photobucket.com/albums/o314/Goldenreed911/clouds.jpgOr use any other cloud picture that has great texture.
Step 3
File>Open As Layer... your cloud picture. Keep the picture as a Floating Image then Layer>New Layer. (Doing that will allow you to move the entire picture whenever you want.) Move the cloud layer around as much as you want, just move it to where there's a good amount of texture. (I moved it so I could see a lot of the bottom left corner of the cloud picture.)

Step 4
Select your cloud layer and set it to Screen. (If you can't figure out how, Dialogs>Layers, click it, then click the drop-down menu and set it to Screen.)
Step 5
Take any soft brush and carefully erase away some of the cloud layer off of your render or anywhere else that looks bad. (You don't want to have it covering a whole bunch of your render, just a little bit.)

Step 6
Do an Edit>Copy Visible. Paste it, then Layer>New Layer.
Step 7
Dialogs>Gradients and select the German Flag Smooth gradient. Then go to Filters>Colors>Map>Gradient Map. Your sig should now be all red/fiery looking.
Step 8
Set that red layer to Multiply. Now turn down your opacity a good amount. I set mine to 25.0 but you can do whatever you think looks good.

And that's it! If you have ANY questions, please let me know either here or in a pm. I look forward to seeing some results.
Other results I've made:


