Draw a '3D' pyramid in perspective

Judge by eye how far along the vanishing lines you think the back edges of the pyramid ought to begin, and draw a line from there to the opposite vanishing point. These lines form a diamond shape - where they intersect (cross) is the back corner of the base. Then draw the diagonal lines connecting the opposite corners as shown. Though they might be nearly at right angles, the important thing with these lines is that they connect exactly to the corners - they are NOT supposed to be parallel or at right angles to the horizon line (though they may happen to be so).