Settings are more than just borders and countries and names on a map. While all those things are usually required for a setting, they’re really only needed as containers for culture. That term is an exceptionally broad one, because it covers a great deal of topics, from the world-shatteringly important to the seemingly trivial. Music is culture, card games are culture, how a given people remember their history is culture. Who they regard as friends and enemies, how they think about abstract concepts, their attitudes towards change, technology, magic (if that exists in your setting), how they choose their leaders, and how they fight wars, all of these things are culture, and a thorough worldbuilder will want these gaps filled in eventually, and for good reason – because ultimately, well-defined cultures help make more sympathetic and better-rounded characters hailing from those cultures.
This is an important point to make because people are defined by the cultures they live and grow up with. Those cultures will push them towards certain likes and dislikes, towards accepting certain things as more “normal” than others, towards certain modes of interactions with other characters, who will naturally respond based on their own cultural background. A “well-built” culture can provide a lot of needed depth for characters which hail from it – explaining their actions or their motives, and perhaps providing useful avenues for character development. How characters engage (or refuse to engage) with the cultures around them can even make for an interesting story by itself.
Generally speaking, there are said to be two kinds of worldbuilding, top-down – where you start with the big stuff like the shape of continents and the high level history of the setting, and then work your way into the details; and bottom-up – where you start with the details and work your way up to the wider setting when needed. I don’t think either of these terms are particularly accurate. Personally, I try to think of worldbuilding and culture-building not in terms of scale, but in relation to the characters which drive my plot: do I build my cultures first, and then use those cultures to inform how my plot evolves? Or do I set up how my characters are supposed to act first, and then build my setting’s cultures around them in ways which justify the actions the plot requires them to take?
Generally speaking, culture-first worldbuilding is better suited for building shared worlds (like RPG settings), or other settings where you plan to have multiple stories from multiple perspectives, since it allows you effectively to build a world within which you can set stories in with characters already partially defined by pre-built cultures. If you’re planning on writing a self-contained narrative however, Plot-first worldbuilding is usually more efficient, since it allows you to build only the elements you need for your plot, instead of spending time and effort fleshing out parts of the world which your readers may never even see. However, it should be noted that neither of these approaches are mutually exclusive. It’s possible to sketch out national borders while writing out character traits at the same time, or to bounce back and forth, using your characters to define culture and then using that culture to define other characters.
Whatever your approach, however, it is important when building a culture to maintain a certain level of coherency. Societies don’t develop from whole cloth, they’re shaped by their geography, history, and material circumstances. It’s important to take these factors into account when developing how a given culture manifests: a society which lives in the desert won’t have a lot of water sports, a society focused on warfare might have codes of conduct and behaviour centered around showing other people that you aren’t a military threat to them (this is how we got the custom of shaking hands). A pastoral culture might have cuisine made up disproportionately of dairy products, they might measure wealth in the size of herds, and dress in hides or wool. Contrast that to our own society, with its vast networks of trade and heavily industrialised manufacturing, where having lots of things isn’t as much a sign of wealth as having a lot of hand-made things, or things made from materials we still can’t mass-produce.
Of course, this rule isn’t absolute. Societies change as their conditions change, but they still maintain throw-backs and artifacts of their previous conditions. We don’t have absolute monarchies anymore, but Chess still has kings and queens. We don’t rely on the wind to move our ships anymore, but we still talk about someone “taking the wind” out of someone else’s “sails”. Firearms don’t use gunpowder anymore but we still talk about “keeping your powder dry”. We have buildings from previous eras, institutions from previous eras, and statues of previous eras’ heroes – some of whom are not so heroic by today’s standards – all standing as artifacts of what our society used to be, and those have as much a weight as cultural manifestations of what our society is – and how a culture deals with those past artifacts themselves can be a sign of how a society is evolving: during Rome’s expansionist phase, gladiators used to be named after Rome’s former enemies: the “Samnite”, or the “Thracian”. But as these conquered people became Romanised, those stereotypes became seen as outdated, or even offensive, and those types were replaced by new ones.
There is also the fact that cultures don’t exist in a vacuum either. Even at the very beginning, societies traded ideas and goods and people with each other, and many of those traded goods and ideas become parts of the societies that trade in them. That’s how Italian cuisine adopted noodles from China and tomatoes from the Americas, or how the rulers of the Russian Empire used to title themselves after the Roman Caesars; or how our modern English-speaking societies, heavily dominated by the cultures of Western Europe, still uses arabic numerals, plays Persian board games, eats foods originating in South America, and speaks with loan-words from languages all around the world. This too, can be an important part of filling in your culture, because it doesn’t just show who the peoples of your setting are, but also who they interact with.
These are all general guidelines, of course. I can’t tell you how precisely you should build your own setting. However, it should always be remembered that this sort of thing is always worth doing. That every pastime and political alignment, every folk song and colloquialism, every religious sect and piece of clothing that we take for granted in our own world was also once nothing more than an idea in someone else’s mind. In your own worldbuilding, every single one of those things is an opportunity to do something different, to exercise your own creativity, and to make your setting a more vibrant, textured, unique, and fleshed-out place.