Using GIMP 2.8.4, the latest. My image is 3000x3000 and prints at 100 dpi.
Then, I duplicated that layer and made the first one invisible by removing the little eye beside it.
Then, on the duplicate, called Fog Light with a mode set to Lighten Only, I scribbled haphazardly not knowing where I should with my mouse using the Paintbrush Tool at 100 px. The color was fffdcc, a very pale yellow. It looked... Well, like I just described. A bunch of yellow lines everywhere.
I didn't bother to do the same with black because obviously, I'm not doing the right thing at all.
Here is the update: http://img6.imageshack.us/img6/7528/shadowyrm8.jpg
Yes, it looks ridiculous, but I wanted to illustrate how little I understand the instructions given.
Also, the default filter needs some help. The fog should be thicker in the low spots. So, I'll have to do something about that. This was just a quick test. The mist is coming from the room with the water (bottom row, very middle).
Further advice would be greatly appreciated, if anyone has the patience of Job to help such a hopeless newbie.