Here's how I work when creating an illustration. First, I generate lots of ideas in thumbnail sketches. I try to not get stuck on the first idea that comes to mind, as better ones may come up after some thought. The thumbnails are small versions in the same proportions as the final illustration. Time is well spent creating thumbnails. Usually there will be more than one idea that makes a good composition, and that can be developed too.
Next, I paint a value study in the same media I plan to use for the illustration.
Then I paint two or three color studies using the value study for a guide.
And finally, I make a pencil sketch, sometimes ink it with waterproof ink, and paint the illustration. A few changes can be made to composition at this point, but not too many for it to work.