The ‘Big picture first, details later’ rule has never worked well for me- Probably because it’s never seemed much fun. Though a more exploratory approach has its limitations and downsides too, I’d much rather begin with an interesting passage, and see where that leads.
Here, as usual, I’ve immediately begun developing detail: Deepening shadows with a 6B charcoal pencil, softening them with a Q-Tip; using a sharply pointed kneaded eraser to define shapes. The cleaner the eraser’s point, the brighter the highlights it will lift out, so for prominent highlights like the breakaway section’s edges and corners, I reshape the kneaded eraser often to maintain a relatively clean point. A dirtier point works well for the tones like the breakaway’s top.