What Tofu is asking you to do for the first 10 boxes is to start by placing the actual vanishing points on the page, as actual dots, then as you add each line to define your box, actually have it converge towards one of these three dots. While this will basically give you no choice but to draw boxes with very dramatic, rapid convergences (since the vanishing points will be right on the page and not far off the edges of the page), it will help you focus more directly on how your lines are converging.

Then for the other 10, you'll draw your boxes as usual (without actually drawing the vanishing points on the page, which will allow you to choose between shallower foreshortening where the vanishing points are far away and dramatic foreshortening where the vanishing points are closer), while retaining that focus on how your lines are converging with one another.