We have all the modern aids to help us by way of reference material, examples, photos etc. etc. But all he had was his sketchbook and memory.
I like to limber up with a sky and remember that Roland Hilder used to say we need to paint a sky each day. Good advice if you can keep it up.
However, here is my sky from this evening. I used a photograph for reference because believe it or not, we have had no clouds for a few days now. Well, at least, not any dramatic clouds or skies.