This should grow from within yourself. Read a lot of design docs and blog posts and articles and books. Which ones did you like? Which were confusing? And why was that? Was it the language complexity? Length of explanations? Diagrams? Too much / too little reasoning? Did you feel that the writer did a good job of picking you up were you were and brining it to a new state which now was enriched with their idea?

Constantly ask youself whether you liked a particular piece of writing and that will over time shape your understanding of what's good and what's not. Note that that's not entirely objectively quantifiable and people will have different tastes. That's also why it's hard to have a "good examples" archive because, just like with code, that would immediately people to start debating. But there is a certain core of properties most people can agree on.