it's different for everyone, but it's what you do to feel comfortable... i mean if you feel that you are just placing things for no reason than to just place them.. try starting on the other end.. begin with thinking up a creation myth.. doesn't even need to be true, but a creation myth will give you ideas on how some things are formed... putting a story behind a geographic feature will give it a purpose, while giving it a legend you can use later in your cultures and such...