Civilization

Once you've established your home world and other important planets, it's time to establish significant nations, whether they're bound to a single world or part of a larger interstellar coalition, and then define major ports of call. When it comes to designing a setting's cultures, you might want to focus primarily on areas the party is likely to explore first. Doing so allows you to establish the details and depth of one region's peoples before expanding out to address others. That isn't to say you shouldn't have ideas about the cultures beyond your starting settlement—it just means you don't need to decide every detail of every culture all at once. As always, you don't need to demarcate every planet in the solar system or indicate every station, city, and spaceport. Keep your focus on what you need for your story and your adventure—leaving terra incognita can lead to stories down the road as the party ventures further from home.