I have seen a couple of renders showing the kind of fiery red clouds you are after but have never made a scene like that myself. I have however produced similar results while experimenting with the position of the sun, so have an idea what to do.
After seeing your question I had the idea of whipping up a quick example and posting it here. Nothing fancy, sun on or just below the horizon and a cloud layer. I find that there is more than a little luck with clouds so tend to follow three basic steps:
First set the basic parameters: Feature Scale, Smallest Scale, Density, Edge Sharpness, Layer Height and thickness. The more you work with clouds the better feel you will get for these basic settings.
Keep an eye on the preview and hit the Random Seed button in the Density fractal until you get the sort of coverage you want. For most scenes the camera position is determined by the terrain so you will need to wait until the clouds come to you.
With the basic distribution set you can then fiddle with the density, coverage adjust and all the other settings until you get the appearance you want.
I got something I liked quite quickly but it has been one of those evenings with lots of distractions so the final adjustments have taken ages. Anyway here it is; this is only a low quality test render, I have a higher quality one running now and will add that once it has finished. I have also posted the tgd so you can see the settings.
