This video demonstrates the process for drawing prospective shadows using a light source. Start with the shadow vanishing point which is directly below the light source on the horizon line. Draw lines from the shadow vanishing point to the front three corners of your box. Next draw lines from your light source to the top three corners of your box. Where the shadow lines intersect, that area defines the shadow area and can be filled in for shadows. The shadows, of course, will line up with your vanishing point as well. The video goes on to show another example in which the shadow vanishing point is below the eye level or horizon.