Paying attention to proportions and shapes will help you to capture the personalities of these small critters. After you have established the structure, use different pencil strokes to add texture, creating the fur.
The basic shapes and lines of some subjects may be hard to visualize, especially animals like squirrels. One method to find the basic lines and shapes of a subject is to tape a sheet of tracing paper over the reference photo and trace the basic shapes of the subject. The lines and shapes of the tracing can be used for observation when doing the structural sketch.