Honestly a lot of Fantasy writers are really bad at creating a fictional culture that stands up to any scrutiny... 
The common problem being that they tend to base everything in something like how that culture does wars or magic, but overlook the basic everyday things that tend to grow into a culture.
When making any culture, I think a good place to start is the hierarchy of needs:
So, we start from the bottom. Any culture that exists started out as a bunch of people who gathered together to better fulfil the needs in this pyramid. So they'd have started by getting hold of whatever they can eat and drink and building whatever kind of shelter they can and making clothes out of whatever materials they could to make comfortable places to rest.
So first questions: Based on the environment they live in, be that a swamp, a desert or a space ship that mines asteroids.... where do people sleep, what materials do they wear, and what do they eat and drink?
So maybe my fantasy culture live in... a mountainous land largely covered in forests with a temperate, wet climate. So we immediately know things like: "They have access to a lot of wood and stone" and "They don't have a lot of flat, arable land for growing wheat or rearing cows" and "staying dry and warm are likely key issues"
Okay, so main clothing material: probably wool, since sheep (or fantasy animals similar to sheep) can live in mountainous regions. knitted wool is stretchy, so now we have a possible clothing aesthetic: tight-fitting wool tunics and leggings, and probably deerskin shoes or boots.
And we know their houses are made of stone or timber, not say mud or clay bricks, and every house has some sort of fireplace in it.
Main foodstuffs are probably some sort of hardy grain similar to oats, meat, fish, and foraged roots, leaves, berries and mushrooms. They likely smoke and pickle food to preserve it (because drying is hard when it's wet). So now we know what the "traditional cooking" is like for this nation. They eat smoked fish rolled up in oat pancakes and pickled eggs, and their idea of a "fancy meal" is probably roast venison.
So, next step up is safety. They're in a mountainous region with lots of stone, and they know the forests are full of predators and bad for visibility, so they probably live in hilltop towns that grew in and around some sort of a walled fort. They're likely best known for their archers, and since they have a lot of wood and probably do lumber work and hunting, let's say they favour axes as weapons, plus throwing spears and bows. They train everyone in archery so they can call up a militia when needed, and then they have a class of warriors trained to use spears, axes and shields. They likely don't use cavalry much, because the ground is too steep.
And now the next step: Where do they hang out? People will look for places to hang out as soon as other needs are met so... Okay, outside is kind of cold and wet and there are bears and things, so being near a fire is likely a thing people want to do. So when at home, probably people hang out around the hearth, and then when they go out, let's say they all gather in big "fire halls", which are basically just big buildings where there are one or more big fires going, and everyone hangs out telling stories... and let's say maybe they roast food around those fires, and make cauldrons of hot drinks, that sounds pretty good!
Perhaps there's also a temple or church, because people need to gather for things like births, deaths and other events like weddings or coming of age ceremonies. Let's say that living in forested mountains, these people worship a storm god, who makes his anger known through acts of violence, but also smites their enemies and rewards good behaviour with bountiful forraging or sending deer for them to eat. Deer antlers look a bit like lightning bolts, so that makes sense, right? So the priests at the standing stones (open to the elements to be closer to their god) wear hats with antlers on, and lightning bolt symbols are sacred.
Also related: This is the stage where your people probably have things like a name for their town or even country, a flag or crest, a local accent, dialect or language and perhaps some distinctive style of dress that makes them all feel like "part of a community". It could be that there's some kind of local plant that makes a dye of a very distinctive colour, so anyone who sees people from this community is like "I could tell where he was from because he had a sky blue tunic, wooden beads braided in his beard and an ornate axe at his side".
Okay, next step up: How do you advance in this society? Well, we know there's a warrior class. Let's call them the "Hearthguard" (because this is a society that thinks of the fireplace as the heart of society). So can you become a Hearthguard by training? Or is it a hereditary position? Can anyone rise through the ranks to be a commander, or is that reserved for nobles? This will have a huge impact on our society... so let's say that anyone capable can be a Hearthguard, but the supreme commander of the hearthguard, who owns all their equipment is the king or queen.... however, in most cases, armies are lead by an appointed "Hearthlord" (because kings aren't always the best fighters), which is a meritocratic position, and so the thing a lot of little kids aspire to be.
Outside of that, let's say they have currency: gold and silver, and outside trade (probably mostly ores, wool, meat, furs and lumber), which means a person can potentially become wealthy and powerful through trade. So now we also have powerful merchants or merchant families who perhaps try to influence the king or queen, and it might only be the fact that the King or Queen owns all the weapons and soldiers that keeps them from just staging a coup! Interesting!
And finally... ART! Our culture is safe, warm, fed, rested, they have community, they have ways to advance and that means they have time to create art! They have lots of stone, so maybe sculpture? Lots of wood, so maybe intricate carved furniture? Lots of wool, so tapestries or fanciful hats decorated with feathers from the forest birds? They can probably make drums from animal skin, wooden flutes, string instruments, and they have a rich "story telling" tradition, so you can now have a lot of fun coming up with all these details that make it feel like a distinct culture.
And there we have a fleshed out and very real feeling community. If there's magic or higher tech, the same stuff applies. Wizards would still need to eat and sleep, want to show off their standing in the community, hang out with other wizards etc.
