5:19 PM, Friday June 26th 2020
Generally speaking, applying subtractive construction - especially in this kind of situation - requires a very strong grasp of how your forms exist in 3D space. Since the demo video is an older one (I'm continually refining my grasp of how to explain and convey the concepts in the lessons, so it was made before I really solidified this stance on additive vs. subtractive construction), it takes that somewhat for granted and I apply subtractive construction there without considering the fact that students really shouldn't be expected to be able to apply it that way themselves.
So here's an alternative approach for this kind of construction that relies purely on additive construction: https://i.imgur.com/97JMPRm.png
As you continue to practice additive construction, your grasp of how these forms exist in 3D space and your mental mode lof 3D space itself will improve, and you will become better equipped to add/subtract volumes from forms.