Help:GM Workshop/World Building/Fleshing Out Your World
Cosmology isn't necessarily important, but I like to start at the most basic groundwork for the setting. To begin with I suggest going through the different planes and deciding if they exist in your world or not. This will further give you an idea of how your world functions. If there is no ethereal plane than certain spells may not work in your setting, or they may use other planes. I like to start with the Material Plane, and determining which planes are linked to it and how. If you don't want to monkey with a new cosmology you can just use the normal one, or choose to completely skip this step. The planes only become an issue if you plan on allowing your players to travel between them, or want to extend play beyond the material plane.
Knowing your Deities is extremely important, is there a single God that everyone worships, maybe people worship different aspects of the same deity. Do gods and goddesses even exist? Maybe your world has none, instead maybe divine power comes from the world itself or from trees, spirits or other things. Either way it is important for you to determine either your own pantheon of gods or use one that is already established. Remember to consider if their is a limited amount of divine power, or if anyone can attain godhood. Maybe the players can become gods?
Maps are extremely handy, it doesn't have to be a masterpiece, in fact a rough sketch in paint will do. But a visual aid is almost necessary if you want to get an idea of where things fall in your world. Having any sort of map will let you mark off borders, territories, factions, cities, landmarks, and anything else necessary in fleshing out your world. While mapping it you may want to generate a set of rules for naming your locations or cities. Choose similar sounds or find a language that seems to give the vibe you want and stick with the naming conventions they use. Alternatively you can use some form of a random name generator and print off large lists of names and use these, or just randomly make names for things. Simply getting a name down will help make the landmark more unique and allow you to expand it later on.
Add as you Go
Don't try to add everything at once, you'll easily get burned out. In fact one thing you can do, is begin an adventure with your players and encourage them to write backgrounds for city or places that you have marked out on the map. As you tell the story you can expand the areas they travel to, and begin to develop a richer environment and feel in your campaign.