The correct answer is
1. Draw a faint horizon line across the middle
of your picture plane.
2. Mark two small vanishing points on the sides
of the horizon line.
3. Draw a vertical line well below the horizon line,
but centered between the vanishing points.
4. Connect the top of the vertical line to the
two vanishing points with straight lines.
5. Connect the bottom of the vertical line to the
two vanishing points with straight lines.
6. A little away from your vertical line, draw a pair
of lines parallel to your vertical line that join the
tops and bottoms of the vanishing lines.
7. Connect the top and bottom of the right-most
vertical line to the opposite vanishing point on
the left.
8. Connect the top and bottom of the left-most
vertical line to the opposite vanishing point
on the right.
9. The two top lines will intersect, forming the top
of your box. From here, draw a vertical line
down to the intersection of the two lower
vanishing lines.
10. Now that you have constructed the box,
erase the vanishing lines outside the box.
Explanation: I'm taking this class on Plato and I used my notes. Your welcome :)