Ultimately what you've been introduced to in the texture exercises of this lesson are really just the application of the same principles Drawabox focuses on, but at a smaller scale. So as you continue to work through the course, you will find that your understanding of how the forms you're drawing exist in space, and how they relate to one another, improves. This will reflect back in your textures, which also rely on understanding these spatial relationships. That in combination with further mileage and practice of these concepts as applied to texture in your warmups will continue to push you along, and I by no means expect you to have a solid grasp of it here. Just a point from which to start.

As far as other resources go, if you're interested in exploring different ways to approach rendering in ink (which is actually something we do not tackle at all in this course, in the interest of staying as focused as possible), take a look at Alphonso Dunn's Pen & Ink books.