1 ) Build your portfolio of clouds by scouring free stock photo sites or taking a walk on a cloudy day with a digi cam. Try to find ones you can easily isolate on their own as shown. The higher the resolution the better the result For Now try this one –>download<–

2 ) In photoshop change the Image->mode-> to Greyscale to make it black and white.
Pull up the Image-> Adjustment panel > Levels to increase the contrast and start isolating our cloud.

3 ) Take a soft edge black paintbrush and ensure all the edges of the image are black, as well completely isolate our cloud in the middle of the image. Next Invert the image from the Image->Adjustment drop down.
As you see below after inverting our black cloud is not touching any edges, everything white in the next step will become transparent

4 ) Last step for the brush is to save it as one, hit Edit->Define brush preset. Give it a name to identify later.
5 ) Here is a sample image where this brush would be useful in adding some interesting detail to the sky.
–>download<–

6 ) Open your Window-> Brushes panel and with your brush selected find the name you choose in the Brush presets, it’s usually at the bottom of the list. You could play with the settings further to perfect the scattering and such but i prefer to place each cloud where i want it.

7 ) In your sample with the clear sky make your selection of the blue area using colour range, magic wand or path tool so your not painting on the buildings. Keep that selection active while painting. You may wish to create a new blank layer at this point to leave things editable.

8 ) Select the colour white and begin placing each cloud as you like. I used a mix of giant 1000px clouds at low opacity with a quick button press. And others with a lower size at high opacity and clicked a few times on the same spot for a really bright cloud. As seen in the brush panel you can squish the brush vertically and flip the x or y axis to achieve a new cloud variation. OR load up an entirely new cloud brush for even more variety.

This tutorial allows you to have absolute control over colour/placement/opacity and size of every cloud with high accuracy and high quality. You are only limited by your library of Cloud images.

*Worth noting* - Try swapping the cloud image for things like bubbles, raindrops, Star feilds etc.. for other useful brushes.

Enjoy!