The first step in creating a civilization to populate a created world, is to determine what level of technology they are at, or if they use magic to do things. This will largely depend on the story or game under development. Is the story a mediaeval quest? Then the civilization could have a mixture of mediaeval technology and magic. Is it a modern quest? Then a more contemporary level of technology and no magic is required. Perhaps the story involves a modern witch or wizard, such as Harry Potter. Whatever the story, the characters are not in a vacuum. They are part of a larger civilization and should feel governed (or oppressed) by that civilization.
Once the level of technology is determined, it’s time to develop the law of the land. The law is the backbone on which civilizations grow. Everyone has to abide by the law, or be hunted by the law. This can create conflict, especially if the law is unjust or overly enforced. The legal system need not be detailed in the story, but the author needs to know the limits that the characters have to operate within the confines of the mores and legalities of the civilization in which they live. To ignore this facet of world building is to risk losing the reader if the characters’ behavior does not make sense in relation to the world around them. A well thought out legal system or code of ethics can make a great deal of difference in the behavior of characters.
It isn’t necessary to be an expert on civilizations. Members of any civilization consider many of the elements of a civilization as common sense. All the world builder has to do for a civilization is create a set of ethical behaviors and morals that go well together. There were very few nudists during the Victorian age. Fashion has a lot to do with that sort of thing, so fashions are something else to consider. It doesn’t have to take center stage, but unless the characters are all nudists, fashion will enter into the story in some capacity. In many Earth cultures, there are taboos relating to how certain members of that culture or civilization dress and act. The world builder needs to know those rules for the world. Again, it may not be germane to the story, but the builder needs to know what those rules are and just how to break them in the story, if that’s part of the plot.
Each civilization will have its own set of rules and laws. Some worlds could have more than one civilization. That is up to the world builder, sometimes simplicity is the best method, but sometimes complexities can add flavor to the work. Proxima One, the world I created in my novel-in-progress, The Accidental Colony is a world with humans on it. They came there from Earth to study the flora and fauna of the planet. There was no indigenous intelligent life, only plants and animals. I didn’t have to create a civilization for that world, but I did have to create one for the future humans, as the story is set in the future. For that, I simply imagined globalization going to a natural conclusion of a single world government. It’s vague, I know, but I needed to know something about the world my characters left behind when they went to Proxima, even though it plays no active part in the story, but I still had to consider it, because it shaped my characters before they went to Proxima.
I have been focusing on the novelist or author side of world building, but the same holds true for RPG world builders as well. If the world and its civilizations don’t make some kind of sense, the whole construction falls apart. Good world building is the foundation on which good stories (and games) grow.