The key? Cloud Brushes. Clouds Brushes are fairly easy to make. Have your camera with you as you travel around. When you see an interesting clouds, take a few shots of them. That's probably the easiest first step ever.
Make a folder for your cloud shots or do some Keywording to be able to find your cloud images quickly after you do your import. Pick a cloud image, bring it into your image editing program. Switch it to a Black and White using whatever you like to use to make the conversion. I'll talk about using Adobe Photoshop Cs6 (since that what I use) but you can probably do the same things in whatever you're using. This is one case where you don't have to use Adjustment Layers since you'll be saving a Brush (B) and then discarding the changes. Invert the cloud image (CTRL I [eye]) and bring up the Levels (CTRL L) dialog box. Increase the contrast by bringing in the two outer slider carets. The result doesn't need to have a pure black or a pure white. We're trying to make a semitransparent Brush.
Grab the Gradient Tool (G). Choose a linear gradient and the foreground to transparent (in the dropdown) option (second from the left). With your foreground color set to white and drag in from the outside of the image. Drag the Gradient in all around the image. You may have to drag as many as ten or twelve times to get the entire outer rim to white.
Once the clouds appear in a ring of white you're ready to make the Brush. The Brush will only be whatever is black/grey (nothing white or light grey). Now create a Brush Preset. Click on Edit/Define Brush Preset. A dialog box will come up asking you to name the Brush. Clouds are pretty standard so Clouds 1, Clouds 2, etc is probably as good as anything. The new Brush Preset will be appended to the brush set you have open. You may want to make folders and save all your Clouds Brushes, your Fire Brushes, your Water Brushes in separate folders.
Once you have your Brushes defined you can choose a Cloud Brush, set the foreground color to something that appears in your "hero" image and start producing your background. I recommend using (and naming) each color and cloud on a separate Layer.
If you were to make your foreground color Red and do one click of the Brush and then change to foreground color to Blue (on another Layer), you'd have some Red, some Blue and some purple. This would be due to the inherent transparency of the Cloud Brushes .