A few tips for thumbnail sketching: Keep your sketches small and simple. They should only take about 5 min each to complete. Don't worry about details, just focus on a pleasing arrangement of shapes and values. Above, I am generalizing mass areas and identifying the areas of lights and darks. I ask myself...Do I want to follow a design structure (or armature) for my painting? Where are the areas of lightest lights and darkest darks? How can I use these shapes to enhance the design of my painting? How will I use these lights and darks (and line direction) to create movement in the painting? How will the viewer's eyes travel around the painting (passage)? Do I have (and want) a focal point?
One of the most exciting aspects of painting for me is finding order in apparent randomness, especially in life and snapshots of everyday moments. For this reason, I don't "stage" my still-life paintings. I find order in what is already there. For example, the garlic above was a photo from the farmer's market near my studio in Fredericksburg. Armatures serve as the backbone structure of a painting, will help create a coherent flow, and are a starting point to determine placement of shapes, values, and color. The disclaimer here... not all paintings need to have a structure to be successful and not all compositions lend themselves to structure (i.e. single-object still life, continuous field compositions, portraits, etc), but many do. Above are a few common compositional structures. Again, these structures are simply guides, not rules, and for beginning painters, will provide a direction in helping you organize and be more successful with your paintings.