When designing and illustrating a picture book, I always make at least one piece of trial art early on in the preliminary planning for the book. This allows me to experiment with media and style while my ideas for the book are still fluid, and I'll be willing to change my mind if I need to. Recently I made an experimental illustration for a project on which I'm currently working. I knew what I wanted the scene to show: A group of mothers, and a group of youngsters, who have lost each other at an old-fashioned country market. As soon as the general idea of the composition came to me, I scribbled it down in my notebook so I wouldn't forget it.
My next step was to start roughing out the composition, using pencil on vellum tracing paper. I always begin a composition with these "place-holder" figures. The place-holders keep me focused on the basic composition, and prevent me from getting caught up in details that are unimportant at this stage.
After roughing out the figures, I started on some of the setting and background.
I didn't like the shape of the umbrellas, so I changed them. Then I worked out a general shape for the tree. At this point I thought that my basic composition was all set. I planned to begin a more detailed sketch on my next working day.