A bank of clouds probably isn’t the best place to start to try to make a Photoshop Brush (B). What you need is a sky full of individual clouds. You know, cloud, sky, cloud more sky, more clouds, sky, etc. It doesn’t even have to be a great shot of a sky. Today’s image isn’t one that could be easily used to replace a bald sky. In fact it’s one step away from the trash heap, but for making a Brush, it’s fine.
The first step is to make it Black and White (B&W). Brushes, in the raw state are black, white and gray. Color comes from whatever foreground color selection you’ve made, by any means you want. (Color Picker, Swatches, sampling, your choice.)
So, now we have a B&W image of a cloud, but the sky is gray. The next step is to crank up the contrast between the blacks and the white. Everything black in the Brush is going to be 100% color when you use the Brush. Everything White is going to be clear (no color) with the Brush. Grays will be varying levels of transparency. Getting that greater amount of contrast is easily done using a Levels adjustment. You can use a Levels Adjustment Layer if you think you might want to go back and tweak things. You can also use a permanent Levels Adjustment (Image/Adjustments/Levels) if you’re sure you can get what you want on the first shot.
Once the sky is a deep, rich Black, it’ time to make it a milky white. Select the entire image (CTRL A) and then Invert the color (CRTL I [eye]). Black is now white and white is now black. The cloud portion is still gray, just the reverse of what it was. This is the time you might want to tweak those Blacks and White with a Levels adjustment again.
Next comes the part that trips up a lot of people. Using the Gradient Tool (G) to feather the edges of the frame. Some folks will feather (apply a Gradient) to the left side and get just what they’re looking for. Then they’ll try to feather the top, only to have the left side pop back to its original state. That because they’re using the Black to White Gradient. The one they’re actually looking for is the Foreground to Transparent Gradient. That’s the one that will allow you to add to rather than replace the Gradient you’ve just put down. Go around the entire frame creating a nice, soft edge to the outside of the cloud. You now have an image that can be made into a Brush.
Make a selection around the cloud and go to Edit/Define Brush Preset. Give your Brush a name and you’ll find it as the last Brush in your default Brush Library. If you’d like to save it to another library you only have to have that library selected when you start.
You can now go nuts using your newly made Brush to do all sorts of creative things. Select the Brush Tool (B). Open Window/Brush and you'll have the Brush Engine available to you. With it on the Brush Tab you can do just about anything you can think of to the Brush. You can torture the daylights out of the thing. The big things, if you'll be making Backgrounds would be to spin the Brush, Flip the Brush, squish the Brush. Just remember to dab the Brush. You don't want to swipe across the "canvas". Just pop, pop, pop, spinning or flipping after every dab. Change Cloud Brush? Add a new Layer. Change color? Add a new Layer. Play with Opacity. Play with Blend Modes. Let I said, go nuts.