The drawing book I'm reading (which, typically, claims to be "for the complete beginner") is full of frustratingly vague instructions:
"Include lines, when needed, for extra definition."
"Perhaps there are places where colours could be built or dark areas that could be lighter?"
"You may also make any additional changes you think necessary."
@peterdrake fwiw - while I am not great at drawing - that was precisely the point at which I stopped using books for learning and started watching artist videos. This isn't natural for me - I strongly prefer books to video for learning. But visual arts unfold continuously over time rather than in discrete intervals. And there's a lot of different right ways to do things depending on what style you're going for. Comparing a few artists' vids for similarities and differences shows a lot. My $0.02