In part 1 of my World Building series, I talked about constructing a fantasy world from the Top-Down, starting with a million-mile view and zooming down to the continents, the countries, the cultures, economies, and so on.
In part 2, I talked about constructing a science fiction universe from the Bottom-Up, starting with people and their stories, then hanging a framework on top of that to support the plot.
In part 3, I discussed a hybrid approach that lets me switch from an idea like Martian colonies (Bottom-Up) to planning out the locations and people living there (Top-Down) as needed.
History - This was always a boring subject in school, wasn't it? That's because most teachers focus on memorization of names and dates. When you think about the people who were there and why they were doing what they were doing, history becomes a fascinating look into what motivates people. Whether you are creating a fantasy world, a science fiction universe, or an alternate Earth, you should think about the history of that world and how it affects the stories you are creating.
Language - Maybe you want to start with an ancient race's alphabet and plan out how the discovery of that affects the people in your world. Starting with an interesting language idea and developing a mystery around it could certainly be the seed for a few plot lines.
Politics - Whether it's a fascist faction in the future or a group of fanatic supporters of a deposed king, creating tension in your world can be accomplished with a number of political ideas.
Races - Creating monsters or races that are unique to your world is a great way to separate your creation from others. How those new races interact with the others can be a great source of tension.
Religion - This is another point that works in fantasy as well as science fiction. Create a deity and a religious group that follows it. Before you even finish drafting those ideas, you should have thought of at least one other group that would be in conflict with them.
I'm sure there are lots of other ways to approach world design. One thread weaved throughout all of these ideas is conflict. Most of our history is about conflict - conflict over ideas, territory, religions, people, and property. Think about the people living in your world and what conflicts they would have with each other, and build on that. Even Utopian stories are really about the conflicts going on under the surface of the society. I don't really want to read 400 pages of people being happy and having everything they want handed to them.
There's no way anyone could craft a world as diverse and detailed as our own Earth, and there's no way I could walk you through the entire process of creating a world in four blog posts. But I hope this series gave you some things to think about as you set out on creating your own fantasy or science fiction world.