This is the tricky part. Drawing the back, hidden sides of the box. You need to draw two sets of vanishing lines. One set goes from the right-hand corner line (top and bottom) to the LEFT VP (VP1). Another set goes from the left-hand corner line to the RIGHT VP (VP2). They cross over.
Make sure you don't try to make any lines meet, don't draw lines to any other corners, and don't worry about any of the other lines they might pass through. Just draw straight from the end of each back line to its opposing VP, as in the example above.