Redwings, worldbuilding (in this case) is the process of constructing a fictional world based on what happens, each turn or few turns, in Civ V. Most everything you or people around you do must be explained in your rough notes.
For example, if your capital grows one turn, and a citizen begins to work a spice tile before "Calendar" is discovered, you must explain what's going on there. Are those citizens farmers, discovering small logs of cinnamon as they slash & burn their way through the forest to make room to grow maize (this would work better if you had workers actually destroying that forest tile, it would make more logical sense)? Are the citizens not farmers, but alchemists, studying the spices day in and out for their unique set of properties (this would work better if you started working this tile after you discovered a scholarly-technology such as "Writing" or "Chemistry"). Perhaps the people in that forest tile are shaman, reacting wildly to the foreign taste of cinnamon and coriander (that would work better if you founded the "Oral Tradition" pantheon (giving you culture from plantations), but it had not yet been implemented there).
Everything needs to be explained, and Civ is full of resources. If you research a tech, you must explain how that came to be. Did finding the wheel have any connection with the artisan pottery skills developed in your city hundreds of years ago (allowing citizens to create a potters wheel)?
If a religion is developed, you must explain why it was developed then (a great prophet was expended, which you must explain) and what the base of the religion is. For example:
A religion with "Oral Tradition" might start out as plantation slaves retelling their exodus stories in the fields of cotton and silk. A great prophet is born, a woman claiming to be "the White Tower", the figurehead that sent the slaves to your city in the first place. She cries out and preaches to the citizens of your city that everyone must be treated equal, as that was her intention when she helped found the city. She tried to make the city peace-loving and multicultural, but once Machu Pichu was erected, the merchant class became increasingly corrupt and stole away the rights of the non-whites from the city (relating back to hundreds of years ago, when you made the decision to focus on the economy and tile improvement rather than faith).
The White Tower would go on to further emphasize peace (explaining why you picked up "Peace-loving" as a follower belief). Then, depending on which founder belief was chosen, that aspect would be explained in society and how the religion made it so.
If your religion is later enhanced, you must explain why it was enhanced at that time, and if it was a cohesive enhancement or a sect of your religion splitting off. No religion stays the same forever. If you pick up a reformation belief later in game from the Piety tree, that must be explained in full as well. Underground Sect, for example, would cause a sect of your religion to be founded on paranoia and spying through arcane practices. If you got To the Glory of God, however, perhaps your religion rewarded great people by making them saints, creating a bubble of wanna-be-saints to rise in your civilization.
In your notes, you'd go into more detail than I am here. You'd name people, and compare your religion to how it was based off of/is the opposite of others. Say you have six followers on Islam in a border town before you spread your religion there: what is the effect now? Check the religious beliefs in the religious menu to see what the skyline of your town looks like now. If Islam has a "Cathedrals" follower belief, cathedrals were built in this town long ago. Do they remain? Is there religious freedom in your civilization? If you use an inquisitor in this city, that's essentially an Inquisition, or something like it.
Record everything. If you meet a city-state that has jewelry and citrus (you can tell when you look at the city-state contact screen, or you go to diplomacy), you must explain why they have those resources and how they relate to the city-state theme (religious, military). Perhaps the jewelry, which you decide to be flowing golden bands hanging from the nose, is part of the biyearly religious fast, the golden bands are supposed to have the essence of God in them, and therefore feed the subjects with all they need. The citrus may be used to break the fast, an activity just coming up.
You'd write all this, and, if it applies to other civilizations or city-states, post in on our subforum. This way, next time people encounter that same city-state, they can talk about how the religious affairs have evolved, and how certain wars have effected their fasting.
If a wonder is constructed, explain. If a social policy is bought, explain a LOT. If you get the "Republic" policy of the liberty tree, your civilization is now a Republic. That does not mean it is forced to stay one forever, though. If you then jumped over into the Tradition tree and picked up "Monarchy", you'd have to talk about the usurping of the senate by the famous general and self-declared monarch BlbaI can't think of a name blab gab.
Especially once you reach an ideology, explain all social decisions. Universal suffrage? Cultural revolution? Lightning War? All of these came out of something, some internal or external decision on your civilization's part. Was "Order" forced on them after the powerful Siam empire decided to undergo a cultural revolution? Does their religion preach "Autocracy" (this would make more sense if some war-related belief was chosen in the past).
It's quite easy to do this, just think of the causes and effects of everything you do. Keep checking all the menus and lists you can find, the "Demographics" section, everything. You can ask around in the forum too.
"Civs of the southern continent: Anyone adopting 'Liberty'? Why?" or,
"After the Russo-Japanese war, Russians have grown fond of spices and truffles, discovered when the Russians held Satsuma for a decade. The traders of Moscow hope any foreign transports would be willing to trade, exotic foods for a hefty fee. Anyone up for it?
Keep writing and evolving your world. Talk about every Great Person, Golden Age, Policy, Tech, Luxury Resource, and everything else you can find or create. Make your citizens change, see your nation evolve.
If you have anymore questions, ask. I'll start posting more tutorials soon. I know this was kinda confusing.