8 of 10
Now you simply have to draw a vertical line from the where the lower two vanishing lines cross, to the intersection of the upper two lines - the red line in the example. Sometimes this can be tricky - the slightest of errors can make them a little off center. If this happens, either start again making your drawing more accurate, or make a 'best fit', keeping your line vertical and fitting it between the corners as best you can. Don't just join the corners with a tilted line, as that will make the box misshapen.
More on Perspective Drawing