Working with solid forms
It's not uncommon to see artists starting with a loose, vague sketch to explore their drawing before committing more firmly to their lines. That is not how we will be approaching things here. That's not to say it's not an entirely valid approach to drawing, just that what we're doing here is specifically an exercise in construction.
As mentioned in previous lessons, construction is accomplished in successive passes, starting dead simple and gradually adding more forms to build up complexity.
At no point are we ever 'sketching'. Instead, every single pass should end with forms that feel solid to you - not like flat shapes pasted on top of one another, or loose collections of strokes on the page. Think about how these forms relate to one another, and add contour lines where you feel these relationships or the illusion of form need to be reinforced.
Try to perceive these solid forms as real, weighty objects in a three dimensional world, and look beyond the fact that you're drawing on a piece of paper. The page is not a flat surface - it is a window into a boundless world.