Painting with oil paints can be desirable for a painting that needs a lot of layers and manipulation, like clouds. While painting clouds, remember that they are perceived to be white, but they are actually reflective of all colors that are around them. The long drying time can be used to your benefit to create a painting of clouds that is beautiful.
Wash (paint a layer covering the whole space) a warm glow over the canvas.
Layer light flowing curves in blues and yellows that are muted.
Manipulate the shapes with brushes, knives, sponges, cotton balls, rags and more.
Add texture with some of the tools you have used to manipulate the shapes.
Work in areas of the clouds. Use soft strokes at first, and then develop to shapes into more robust and bold shapes.
Let the paint dry. Coat with a varnish to protect your painting.
Use color to your advantage. Look at pictures of skies, and see how the colors from buildings and landscapes show up in the reflections of white. Use colors of a sunset to underlay the tones in your painting, and let them shine through.
Paint in shadows that are reflective of what is around the clouds.
Oil paints are very slow drying, so the big advantage is that you will have a lot of time to manipulate and modify the painting.
Mimic the shapes and dimension of real clouds. Use strokes that give a three dimensional look to your clouds.