Nothing magical; just front and back of the midpoint between the eyebrows until you settle on a point where you get the sensation. That is why i called it a "physiological trick".
Yes, but not fully cross-eyed, there should be no strain, you are focusing on a point within the "blackness" when your eyes are closed. Start with the midpoint between the eyebrows and keep your whole focus/attention on it and nothing else i.e. let go of everything with no other thoughts. Slowly move the focal point back and forth until you literally feel a jolt/dropping sensation.
To me focus means to not move the eyes but to bring the attention in that area and be aware of the sensations there. For example, just bring your attention on your left foot big toe at this moment and suddenly you are now aware of your big toe which was not in your awareness otherwise. Just keep your awareness here and you're focusing on it.
There is a important point to be made here; viz. The physical activity leads the mental focus/attention. In your example, flexing and relaxing the left foot big toe makes it far easier to bring the focus initially on to the activity/sensation at that point and then expanding it to overall awareness.
In the exercise i mentioned, you physically focus the eyes at a point in the blackness and the mental focus/attention follows it simultaneously.
I wonder how the somatotopic map relates to proprioceptive awareness. I've kind of implicitly assumed that executive functions like the attentional spotlight rely on connecting to that area to accomplish conscious proprioception, does that book go over how it relates?
You will have to read the book to find what you are specifically looking for.
The basic idea is that the space around your body to the maximum distance reachable by your arms/legs called "peripersonal space" (https://en.wikipedia.org/wiki/Michael_Graziano#Peripersonal_...) is integrated into a larger body schema maintained (partially) in the somatotopic map and its homunculi. Proprioception is part but not whole of it and how it might be regulated by higher executive functions like focus/attention is AFAIK not clear.
