You may want to check to make sure that you are placing it in the proper position, otherwise there will be distortions. You then create a vertical line descending directly from the light source or light vanishing point until it intersects with the surface where the shadow is being cast. This point will be your shadow vanishing point and shadows will radiate out from this point. Then send shadow convergence lines through all the corresponding bottom corners until they intersect with the light lines. This will mark the extreme edges of your shadow.
Light radiates out in all directions.
If you have a light source directly above an object, then your object will cast shadows in all directions. When the light source is the SUN or MOON, the shadow vanishing point is on the horizon directly below the light source.