Getting more advanced
This bit here is not for those of you going through the exercise for the first time as part of your lesson 1 homework. As I mention in Lesson 0, once you're done with a lesson, you'll continue practicing these exercises as part of a regular warmup routine.
This exercise is especially good for warmups, and I'd generally recommend doing it over the simpler ghosted lines exercise for obvious reasons - it's the same thing, but with more purpose to it. On top of that however, it's a particularly versatile exercise that can be modified to start incorporating perspective concepts as well.
While at its most basic form, I don't want you worrying at all about perspective, as you move forwards through the lessons, you can start to envision these planes as being three dimensional rectilinear surfaces - that is, each plane represents a rectangle or a square floating in space. The third and fourth steps, where we construct the diagonals and the cross can be treated as being a subdivision of the plane (the X to find the center of the plane in three dimensions and the cross to bisect it). The cross will of course require some estimation to find the center of each edge in space, but it's a great thing to practice as you move through the rest of the Drawabox lessons.