To say it’s harder than it looks would be an understatement. Your story is more dependent on it than you think, especially in both fantasy and science fiction. You have world that are so complex like in Game of Thrones, more simple and based on this world like Harry Potter, and then complete universes like in Star Wars or Star Trek. It’s always so beautiful to watch how the setting influences the characters. But, how do you build it from the beginning?
Ironically, much the same as you do a book.
Outline the big stuff. Is it all in one country? Several? Know where they are in relation to one another as far as north, west, east, south. Simple enough, right? Next step is to put that into Microsoft Excel. Set each country to a different color, and put a key to the right accordingly. (I’ve put a picture as reference from my latest book for what I mean as far as using Excel, minus the key. I don’t mean to have it so detailed and laid out in the beginning.) This outline should be completed while you are outline your main plot.
Once you have your basic outline of the main plot points (Again, I’m going to have to suggest Structuring Your Novel by K.M. Weiland if you’re having problems.), then it’s time to delve into the culture. If you already have a clear image of your characters in mind, this part should’ve technically been at least part way done. Now it’s time to go more into detail. Make sure to put sub-races within the same culture. Look at America, Germany, France. There’s people of every skin tone. If your culture is ‘pure’ like the Japanese, why?
Do they celebrate religious holidays? What different classes does it compromise of? How is violence looked at? Gender equality? Religion? Liberal or conservative? Monarchy, democracy, anarchy? Animal life? What’s their main source of food? What’s the weather like there? How does that affect your plot? What’s their take on honor? What do the buildings look like? How do they view art? Do they have their own language? What type of job does a ‘commoner’ have? What sports do they play? All of this might not make it to your book, but nonetheless it works in your favor to show a well-rounded nation.
Now that you have your outline, time to go into the subplots. If you already have some in mind, how’s your map effected? Do you need to put in mountains? A lake? River? Sea? If you’re struggling with creating subplots, refer to the last paragraph. If it’s all within one or two nations, delve more into it. How could the answers you come up with affect your protagonist? If you’re going across many countries, create a whole new one. Make it drastically different than your protagonist. What’s the worst type of culture they could stumble upon? Now, create it and add it into your map.
Look at your excel sheet. Think about natural geography. What’s missing? Add it.
Once that’s complete, look at the new countries you’ve created. How do they affect your general plot? If there’s a war, why aren’t they helping? Could they help? How would that change things? What’s their relationship like with the surrounding countries?