Once I have a grasp of the culture itself, I like to look at the culture's neighbors and work out how they will relate to one another on a political, governmental and religious basis. What do they have that their neighbors might want? What do they believe that their neighbors might not? What do they have to do in order to communicate?
After I feel I have a grasp on the current political situation, I look to the past. Where did this come from? How did it evolve? What led to the current situation? I write as much of the history as I feel will be important to the book, and probably more since I like to fill that in instead of leaving it bare bones.
I fill it in because often myth, legend and folklore play a large role in people's lives, even today. Some of us love myth and devour it because it speaks to us, we tell our children folktales although they're not always the same folktales our ancestors told one another. We have our own legends as well, people who do interesting things or win against the odds, or do something really well.
These things have endured for a reason and I think it's important to the story. These things illustrate the lessons people teach their children, their views on the world, their superstitions and their fears. The provide idioms, expressions, and analogies. They shape the way these people communicate with one another, and they can be used in that to really create a sense of the setting for the reader.
Once I've got the government, politics, religion and history down, I turn to the details of clothing, expression, use of materials, architecture, entertainment, etc. These are things that don't always come through in my first drafts because it's much easier to figure out where they fit in once the book is finished.
I'm always looking for ways to include my world building in the story without bogging it down. Details are a great way to do that because if the same detail is used five or six times a book, many people won't even consciously notice it on the first read through. Those most likely to notice are the ones that tend to like details (hence them noticing them). Of course, that doesn't go for phrases, which I think people pick out a lot quicker.
Once I have an understanding of these things about the culture, I start putting names to things. Governmental positions, religious position, people who use magic, anything that needs a name that is specific to the culture from which it comes.
I try not to give names to things I can more easily explain by using a well-known word, and I don't often name things in a culture's language because I think they can be more easily understood with a descriptive name. However, I may call magic users "casters," "runists," "spell weavers," etc. I choose something that fits the story, the culture and the culture's view of that thing.
When I have things named, it's time to write!
Building Cultures: Part 1
What are the most telling or important aspects to you? What elements help you define the most about your culture?