Keep them loose and gestural, it helps you move quickly from idea to idea! Here's a picture of my thumbnails. They probably don't make a lot of sense to anyone else, but it helps me work through my ideas and placement.
You can see how the drawing is worked out and clearly defined. You can also flip back to my thumbnails and see how the placment and sketches of those effected the final drawing.
Some process cutting as seen in the videos:
Here's a detail shot of my final piece. This photo helps show the depth under the frame.