Worldbuilding is the art of creating an alternate universe where the rules of present-day Earth life don't apply, and you have been appreciating that art for as long as you've been reading or watching science fiction.
Some worldbuilding is epic in scale, and requires thousands of people: the Star Wars universe is like that, if you think of all the people who have helped create the movies, books, art, TV and games from that world.
But other worldbuilders work alone. Ursula Le Guin wrote several novels set in her "shared worlds" universe without any help and without spawning any spinoff tales by other people.
But worldbuilding doesn't have to be something that just the pros do. You can get in on the cool create-your-own universe action any time you want, and fast. Just follow our five simple rules for whipping up a universe in your spare time.
First off, though, let's consider your motivations. Why are you building a world? Is it just for yourself, while you are bored at lunch? Or do you want to use it for the backdrop of a story or game you'll share with others? These questions help you consider what to put in that world. After all, if the world is just for you, why not have it populated with masturbating alien slave men? But if you're planning to invite other people into your world, you might not want to overshare your fantasies quite so much (unless you're John Norman, and that, as they say, is another post).
1. Do a little research. Every cool fake world is based on an interesting real one. Want a world with lots of machines? Look to contemporary Japan or nineteenth century England for inspiration. Want a world that's sparsely populated and devoted to farming? Read up on Northern Russia, and find out what kinds of dwellings and cultures exist there. Since this is a quick and dirty worldbuilding exercise, don't spend too long with the research. This is a fake world, so you don't have to be accurate. You just need a few models to base your creativity on. If you like, you can just chuck reality altogether and create a world that's based on dreams, City of Lost Children-style. Or a world based on computer-induced hallucinations like The Matrix. And hey, if you're feeling generous, you can even hire a researcher to do some of this work for you, just like Neal Stephenson did with his Baroque Cycle.
2. Have a few rules. There is nothing more annoying than an alternate reality where anything goes. That's one of the reasons why the mental landscapes in The Cell were so annoying: When our psychologist in that flick used her special machines to enter the mind of the serial killer, suddenly they were in "mind space" and nothing made sense. Sure it looked pretty, but who wants to hang around just looking at crap if there are no ground rules at all? Even if your rule in "mind space" were just something simple like "If you die in mind space you die in reality" (a typical rule) that goes a long way toward making the action in your world more exciting. Rules create problems, and solving problems is exactly what people like to do in a world.
3. Don't obsess over consistency. Look, we're worldbuilding on the fly here, so don't worry about being insanely consistent. That's why you want to have just a few rules rather than twenty gazillion. Otherwise there will be so many problems for your characters to deal with that walking across the street will be like filing a legal brief. You do want a few things to remain consistent, however, so spend five or ten minutes making a list of the most important constants in your world. These could be things like the names of countries and characters, or the capabilities of special technologies. It could even be a list of laws or ideologies perpetuated by an oppressive government, 1984 style.
4. Consider what's good and what's bad about your world. Let's say you're whipping up a completely awful world like the one in Hellraiser or the post-apocalyptic Earth in the Terminator series. It's great to show us a bunch of crushed skulls and people with pins in their faces, but nobody is going to stick around for a pure torture world unless they are Matthew Barney fans. There's got to be something cool, fun, or intriguing about your universe. In Terminator, for example, the cool part of the sucktastic world is the human resistance and its ability to seize control of some of the Terminators.
Of course, you have to consider the opposite as well. If you create a paradise-like world where everybody lives in peace and has biotech that cures all diseases, you need to invent something problematic or wrong to set stories in motion. Even if the problematic thing is just a bad love affair. In Octavia Butler's trilogy Lilith's Brood, we are treated to a world where aliens with complete power over their genomes live without war and cruise through the universe in eco-harmony. Sounds perfect, right? Unfortunately, they only way they can reproduce is by swallowing planets and assimilating other species' DNA into their genomes. And the human population of Earth is their new object of assimilation into the happy peaceful ecotopia. Doesn't sound so peaceful anymore, does it?
5. Create characters who are plausibly the products of your world. Not all worldbuilding has to have characters, but usually it does. Even if you don't have characters, you're likely to have ecosystems, and this rule can apply to those too. Either way, you want the life in your world to make sense. Obviously an agrarian culture won't produce a computer whiz, since it will be devoted to farming rather than information technology. A technical whiz from an agrarian world might invent something like the cotton gin, however.
By the same token, a highly technological and urban world probably wouldn't be conducive to warriors who fight with crossbows on horseback. Sure you could invent some convoluted rules to make that happen, like in Rebecca Rowe's novel Forbidden Cargo where an extremely high-tech society stages elaborate samurai battles in virtual reality. But this is quick and dirty worldbuilding, so I'm going to say you should stick with Occam's Razor as a defining principle for your characters: Go with the simplest explanation for their origins, and make things complicated later. Alright, you've got a few rules. Now start building. I expect to see some seriously cool alternate realities by nightfall.