"A DM's got to know his limitations."
~ Clint Eastwood
Of late, the DM's Advice videos I've been watching have been all about worldbuilding, videos like this from Dungeoncast (2017), this from Acrelatae (2015), or this from Dawnforged (2012). Dawnforged did a bunch of others, based on this one. This guy did a two-parter that I talked about before, this douche puked about it and the boys Pruitt and Jim did a profoundly vague culture dump version.
Then, the whole thing is complimented by an endless parade of people who have co-opted "world-building" to describe a novel setting (which I've also talked about before), giving us videos such as this one, this one, this one, this one and this one.
All these videos have one thing in common. They are unwatchable.
If I set out to explain that we will need design a unique electric drill in order to complete the building of our project, it is useless to explain that the drill will make holes, that it will run on power, that it will need some kind of bit, that it should probably be small enough to carry in our hands, that it will replace the inadequate tools we're using now or that it will be really, really great for the job, once we have it. Those things are all true, but none of it tells us how the drill will be designed, modelled or manufactured. This "advice" is nothing like advice.
We need to start with the question, what is a "world." Essentially, for an RPG, it is a fictional setting catering to the cognitive point of view of a group of players, intended as a causal impetus to behaviour that can then be modelled into the setting's structure.
And see, right there, we are miles from the shadowy non-specifics that most DMs employ in trying to explain what's needed. When I set out to answer the question, I want to deconstruct the problem ... because it in taking things apart that we find out how they work.
Lets examine that jumble of words I've used, so that we understand exactly what's involved. "Fictional." We're going to fabricate the details from our personal vision; some of this detail can be stolen and worked into the frame of our consciousness, while much of it will be re-invented from the gestalt of art media that we've been consuming since being little children. "Setting." The world will be a place, with dimensions, appearance, inhabitants, a momentum of sorts, a generalized agenda of purpose and pattern, probably predictable since we are incapable of constructing anything that isn't heavily influenced by our psychology, biology, habits, socialization and inseparable human biases. "Catering." The world will provide what is needed for the functional activity of the player's perceptions, knowledge, game play and imaginative awareness of what is happening; that is, once the imaginative structure has been constructed, it will be passed to the players in an effective, believable manner that it can be imaginatively consumed. "Cognitive." This will be done with our brains, with the communicative mental action through which we acquire, process and store information, for use in the immediate present or the imprecise future. "Group of Players." Participants; both those who are presently ready to play the game and those whom we have not yet met, who may turn up in the unknown future with a ready desire to play in a setting that cannot, by virtue of the rules of time and space, be specifically tailored to their needs. "Causal." The setting, information and communication of the above must be designed so as to impel the players to adopt beliefs, convictions, intentions and plans that inspire action, or more specifically a response to the created cause. "Impetus." The causality we promote will include a force and energy that will make the response irrepressible and exciting, so that the player will feel immersed by the game's ebullience and lively presentation. "Behaviour." The players will act. "Modelled." The action presented by the players will effect a singular, identifiable change to the setting, the manner in which the game is communicated going forward, the material causes that occur afterwards and the unrestraint of the game's momentum, in a manner that will require the worldbuilder to rebuild the world in an ongoing fashion in real time as the game is actually played.
And that, Gentle Reader, is merely the definition of what we're building.
We haven't remotely begun how to build this.
The task is enormous. It is not insurmountable. Once we can correctly understand what's required, we can go through and figure out a rational, practical starting point, from which the rest of the structure can be built. That starting point is not the "fictional" aspect. It is not the group of players, nor the manner in which we cater to them, nor in the setting, all places where most vloggers expect us to begin. That is because these are the easiest parts to comprehend.
The starting point we want is this: what can we, as DMs, right now, with the knowledge and experience we have, effectively run? What are our limitations? Everything else, outside of establishing this, is pure bullshit. It is worthless to discuss starting a campaign based on a seagoing adventure setting if we don't have any knowledge of sailing. It is worthless to discuss upon what we should be putting our focus on if we don't know what focus we have. There's no sense in drawing lines on a map until we know what the lines mean, how maps work, or what geography is.
So if you have never built a world, and you want advice on how to build a world, start by writing down what you know about anything real, that you can actually touch. Your fictional, made-up setting, based on your imagination, is going to depend 100% on what you actually know about stuff. If you don't know that much, then you're going to be pretty awful at building.
Once you start to know things, or operate inside your knowledge, you'll start to see what else you need to know ... and once you motivate yourself towards deliberately seeking out and educating yourself about things you need to know, you'll be on the right path.
Don't worry. You have decades to learn how to do this. Bite off exactly what you can chew, then start figuring out how to make your mouth bigger.