So here's how I do my clouds... It works for me, so hopefully it will work for you..

1) I start by stamping my image on good quality paper. (I used papertrey)








2) Stamp main image on a post-it, cut it out, and cover image so it stays white. (Basic mask technique)










3) Using post-its again, draw some cloud shapes and cut them out. I always cut at least one of the clouds as a reverse mask (see step 5) so i get a couple of different levels.







4) Airbrush your base sky colour. Don't make it perfectly even, you want some texture (bare spots, spots with a little more blue etc...)










5) Using different shades of blue, start to lightly airbrush using your cloud masks.










6) Don't forget to overlap your clouds, and use lots of different blues. This step is how i get white clouds. if you put the basic cloud mask down, and spray with your lightest blue colour, you'll create a subtle wash colour for the sky, but when you lift up the mask it's white!







7) Place a couple of your cloud shapes off the paper. And keep layering, layering, layering...







8) I'm finished the clouds! Pull off the mask of your image and colour away.