Stormy range - 6x12 in. acrylics on deep cradled wood panel.
First of all, you need to practice with the concept of scuffling, which is essentially using a bit stiffer brush, a little white paint at first and starting in a center point you gently "scrub" the paint around lightly, further and further around until the brush is out of paint. At that point, you should have a pretty fuzzy shape, lighter and fuzzier on the perimeter so that it seems to disappear. Then you simply repeat the same procedure, and again, and again, until you feel that it looks "airy" enough. Try to avoid hard edges.
In the painting above, I used only Cerulean Blue, Raw Umber, and white for the clouds. I kept mixing various tints in order to get the right variation in clouds. I never used black.
At some point in this process you'll have to decide where the light is hitting the clouds and apply more white consistently so that it makes sense visually. So go ahead and practice on your favorite surface.
Now, for my personal technique: I work on wood panels, and since they tend to lack texture like a canvas, the paint slips around a little more (make sure you do coat your board with gesso, but it's not a fix-all solution.) For that reason, note that scuffling is more difficult and you just have to practice a little.
Because I live in a dry climate, I like to use Liquitex blending medium. I don't use a lot, just have a little pool of it ready to dip in. It really helps in the scuffling method and extends slightly the paint wetness without keeping it wet too long like the Open series. I also work a little faster. Note that the blending medium dries glossy so I make sure that after my painting is done and very dry (1 week), I apply a satin varnish to even out the whole surface.
Finally, a great video to watch is https://youtu.be/OIB1Rddh2p8
Hoping this was hopeful, let me know how it's going for you.