So I've been experiencing this issue myself during this challenge. My pen doesn't really change in weight based on whether I'm pressing down lightly/heavily, and kinda stumped me for a bit. But, by just going over the line again once or twice, you'll bleed just a bit more ink into the paper, making it thicker. It's not much, very little in-fact, but it's enough for the subconcious to pick it up and outline the general shape of the box (They actually talk about this during the reading for the challenge).

This is what works for me, though it might not for you. Even if it doesn't, don't worry about it too much. It's just a recommendation and helps visualize better what orientation the box is in. Hope this helps!