Draw a rectangle roughly the size of a postage stamp and draw three shapes in it: the primary character, a key item, and the biggest chunk of the setting/background. It should take less than a minute to draw. Make another rectangle, and move the same three shapes around. This is the thumbnail sketch. It enables you to work out the composition of your illustration without the time it takes to draw details. At thumbnail size, you have nowhere to put detail. You won’t draw eyes, clothes pattern, lines in the grass.
You have to work out what your central point is, how much room your main character gets, and in which direction the viewer’s eye will move. A central figure looks stable; a figure at the side looks dynamic. A foreground shape that’s much larger than others in the scene feels closer, while shapes of a similar size don’t look very deep or realistic.
For example, let’s say the story describes a girl seeing a fox under a garden bench. Your thumbnail sketch might show the garden from far back in perspective with the girl and fox small; you might crop very close to the girl and the fox’s pointed ears; you might use the bench as a prominent foreground shape with the fox peeking out from underneath. The drawing itself doesn’t have to be good. You need to make decisions on how to show the scene.
Some artists resist doing thumbnails because they want their sketch to be neat and finished. They do their first image very carefully and feel attached to it. If the composition feels off, they try adding more detail in the background, more color, more objects to make it better. These won’t help if the arrangement is bad. You’ll get a more satisfying result if you first try a bunch of different thumbnails so that you aren’t reluctant to get rid of an unclear thumbnail because you’ve put many hours into it.
For a full-page spread, the same thumbnail exercise works to get the layout right. Draw a small block to show where the text might go so that you know if there’s enough space in your image to have it. See if your main character or action is right up against the gutter; is there an arm or face pointing off the page? Also, do some value sketching of your thumbnail, blocking in where the biggest lights and darkest darks are. If you can’t tell what your center of interest is when there’s just a few shapes of light or dark, go back to the rough drawing and change it. A good thumbnail doesn’t have to be pretty; it only has to work.
Can you get the main idea across? Is there a foreground, middle ground and background? Do you have room on the page to put in text? Do the thumbnail sketches support what the author is trying to say? Keep the sketches that get the point across best; pick one to enlarge and do your more finished drawing with, but don’t make it nice. It will be more likely that it will be good if you keep it messy so that you don’t get so attached to it you can’t easily move it around or change it.