Step one: Use the gradient tool to create a gentle sky gradient. Depending on the time of day, atmospheric composition, etc, the colors will be different, but for Earth or similar, make it a light sky blue at the bottom, fading to a darker sky blue at the top.

Step two clouds, my method is thus: Acquire some high-quality stock photography of cumulous clouds from wherever (Make sure you obey the copyright or royalty lays/rules). I recommend sxc.hu Now bring them all into the same canvas and place them so they're sort of overlap each other, and form a semi-beleivable cloud formation, go through each layer and CTRL+SHIFT+U (Desaturate) and set the layer to screen, you'll notice lots of squares and other ugliness, that's alright go through each layer again and adjust brightness-contrast until the square edges start to fade out (brightness down, contrast up), making sure you don't make the clouds white out into silouhettes. You may not be able to get everything, Try and place the ones that've lost their square edges at the edge of the cloud formation (see below) and then put others in whatever placing pleases you. ( View Image )

Step three: Further blending. Now that you've got your clouds all desaturated, set on screen, and contrast-adjusted, you need to blend them. On the ones with the clear edges (like the ones above) not much is needed to be done, but on the ones where the sky-part of the stock photo failed to fade completely, you need to go to work with the eraser and smudge tool, I recommend using the following brushes:

Or whatever else works for you. Those brushes work for erasing, smudging and brushing. Remember, clouds have volume, so you need to shade them a bit if you erase stuff and disrupt their natrual shading. Also, try and rotate the cloud layers so that the light all seems to be coming from one direction. Brush on a thin layer of dark color and smudge, for shading, if necessary. Also, try and keep the clouds from overlapping too much and whiting out. If you get a bunch of really bright patches, erase some of the cloud layer so that they dim, and them smudge-blend them. While clouds ARE white, they're not blindingly do, nor do they emit their own light. Never use perfect white when brushing your clouds.

With any luck, now, you've got something like what you see at right. A nice view of the top edge of a fluffy cloudbank just ready for some action. I have a feeling some will say that the steps between the blue sky gradient and this cloud product were sparse, but I feel if I went into complete, step by step detail, all those who follow this tutorial will produce this same result, which would be a waste of time. This way, there's an element of randomness and creativity, required. ( View Image )

This next part will detail the faded-planet effect. This is a simple matter. ( View Image )

First, acquire a planet, I make my own in a separate, complicated process, but there're tons of planet tutorials out there--if you don't know how to make a planet follow one of them. Get you planet out (see right). Please note the totally black night side. While sometimes I like to leave a bit of illumination on the rear, it must be totally black for this effect to work.

Now, desaturate the planet layer, which should be flat (single layer) CTRL+SHIFT+U. You may or may not need to adjust the brughtness-contrast of your planet to get a good feel for it, depends. Use your judgement, I don't beleive I adjusted this planet.

Now, bring the planet into your sky-and-clouds canvas , and set it to screen. I overlapped the planet into the blackness of the tutorial image a bit to demonstrate a bit better what occurs. Adjust levels or brightness/contrast as necessary to get a strong visible crescent. You can see over the blueness of the sky that it has a very good look, much like the moon seen during the day, This will work for any "phase" of the planet that you choose, although I feel it works half-visible or less.

That's pretty much it. The more time you put into the clouds, by the way, the better they'll be. My final-version cloudbanks are usually the product of 4 hours or so of work. and maybe 20 different stock photos, among my own brushing and smudging and what not. Remember that the clouds should be set on screen mostly because they'll take on a color of whatever sky you're using (usually blue) and be more realistic. Clouds , after all are not black and white, they're ver light blue and darker blue.

Anyways. Be sure to experiment, try many different combinations of painting and cloud stock and blending before settling on a final image. Also try and keep it from just being a straight line of clouds, unless you're very specifically going for that. Clouds are one of the most random, fractal and beautifull products of nature. Treat them as such :)