This series of images shows the process I sometimes use in developing the composition and values for a painting. I like to have a strong value pattern of lights and darks, and preparing these small studies helps me keep on track as I work on the final painting. When I am unsure of where to go next (or what color/value to use), I consult the sketch.
I usually start small with a pencil thumbnail sketch to block out the big shapes and values, then proceed to a small color version, sometimes using pen and ink. From there I prepare a larger version, usually in color. I begin to familiarize myself with the subject and note any details I want to add. After all of this preliminary work, I tackle the final version.
Even though I have theoretically resolved everything, I still leave a lot of room for change and for the "painting to work itself out on the paper" as Robert E. Wood said.