perspective sketch of the primary form
Be sure to define the edges and corner points clearly.

Draw prospecting lines
from the edges of the front planes to find the three vanishing points. Extend the outer edges of the form until these prospecting lines intersect at three separate points. 

The Horizon Line Triangle.
Connect these three vanishing points with three horizon lines. You have defined the horizon line triangle that will define (and usually contain) the primary form. 
 

Constructing Auxiliary Horizon Lines.
Draw the three auxiliary horizon lines through each vanishing point and perpendicular to the opposite horizon line. The arcs drawn to get x and x and vp3 divided in half.