I got my ideas for painting sky and clouds from sources such as
Tried and True Trains, Tips for Painting Clouds
Sierra Scale Models, Painting Backdrops With Ease
I bought two shades of blue, with the idea of getting a continuous transition from dark at the top to lighter at the bottom. I brushed the paint, blending the colors as they met. Frankly, I achieved three color bands. It looks better in the above photo than in real life.
You can search the Internet for pictures of clouds for inspiration. I made stencils for the clouds out of cardboard boxes. Some of my stencils are shown at the left. The "wisps" (A) turned out to not be a good idea when the stencils were used repeatedly. I found it very effective to paint some clouds then move the stencil left or right and continue the clouds.
These templates are held close to (but not hard up against) the backdrop. If you hold the stencil about an inch from the backdrop the cloud edges are more sharply defined than if you hold it further away. Then, using an airbrush or spray can, flat white paint is sprayed very lightly. Less is better in this instance. Your clouds will look nebulous and hazy if you just mist it on. Too much and they start to look too solid.
For the storm clouds (below), spray gray paint first and
over spray with white to the desired density.