Use a sponge painting clouds!
When reading about William Turner, I read that he used a lot of sponging for blending and textures. As a beginner, I had some trouble creating believable clouds. But sponging was the eureka!
Method: Make a (blue) sky wash. Wait for 1 minute. Then take a little sponge and start dabbing out the spots you want to be white. Then drop in a little gray for cloud shadows at the bottom of the cloud.
Then, immediately after that, again dab with the sponge to correct and blend the upper shadow outlines. Make sure that the sponge always has the right dampness (find out how) in order to avoid flowing-around water.
Keep the sponge clean, change the corners frequently while dabbing. I use a normal cheap household sponge for window cleaning. Sponges are great! We can learn from the masters like W. Turner!
Have fun, Lorenz!