Notice how the artist works in a general-to-specific manner in this process by first roughing out the lightest and darkest parts of the drawing, then adding the mid-tones, and then getting progressively more nuanced in refining value and adding detail.
An example of an effective contour line drawing is included above. Note that the contour drawing includes lines of varied weight. In some areas the lines get quite bold, and in some areas the line becomes very light. This variation of line weight adds visual interest and creates a sense of depth.