I was thinking about how I go about building my worlds - I've done quite a few by this time, each to different extents. So how is it that one goes about beginning a world? Where does it start, and where does it end?
The beginning of a world should be a story (or the seed of one).
You can start a world without a story, but then you may be taking the risk that you might never find a story in it - and while that's great for some people, it would never satisfy me. My Varin world started with the idea that ancient kings were cast down and became an undercaste that then had to redeem itself (a story), except the regular fantasy explanations weren't satisfying me, so how would that really play out if the world worked in a realistically logical way? The world of Garini (Let the Word Take Me) began with the question of "Okay, you've got a language entirely created from references to canonical stories, so how would that really work and be passed on?" Aurru (Cold Words) grew out of the idea that rank and injustice should grow somehow out of the distinction between warmth and cold. In each of my linguistics stories the story is inherent within the language concept, because the language concept itself is a sort of punch line that the human characters will have to figure out somehow.
Thinking of the story idea as a world seed is actually a useful metaphor, because the intermediate process of world building is like growing a plant. The original story idea sends you in a direction, and then you'll discover it's branched into a sort of cultural (or physiological, or ideological, etc.) dichotomy, and then communities will start to associate themselves with divisions of thought, and then you'll discover patterns of life and behavior for those communities, and start to ask yourself all kinds of niggly questions that branch in every direction. If the world is alive, you should be able to discover smaller twigs and leaves growing from every branch you might care to consider. The other reason I like to think of worlds as living plants is because everything in your world should be connected. The climate influences the housing and the scarcity of resources which in turn dictates behavior and indicates what things will be fought over. This will create winners and losers and people with terrible things at stake. It's all connected. If it seems unconnected - like for example if you have two people whose names don't fit in the same phonological system - then it needs to be connected in a different way. In the case of Varin, I had taken for granted that the undercaste had a different religion from everyone else - and then when I looked closely at it, I realized there was an entire thousand years of backstory behind that. An entire opposite side of their world, which explained some of the divisions I'd created without realizing it. Just the way that people assume that others speaking to them will be engaging in conversation cooperatively, and draw conclusions based on that assumption, I encourage you to maintain a strong assumption that everything in your world has a reason behind it. This will suggest hidden depths that even you can't initially imagine, but which will reveal themselves to you if you consider them closely.
Not every story requires the same depth of world building. I admit I go pretty far with world building in general. I do climate and demographics and all those checklist things, plus language and culture. However, my short story worlds still don't get elaborated to the extent that my novel worlds do. Varin in particular took me (on and off) about 20 years of work.
Now, once you have your tree grown to the size it needs to be for you to understand the story, what do you do? How can you end the process of building and create a story without getting lost in all the branches?
My answer is, find a character. The character, with his or her upbringing, identity, and judgments, will create a microcosm of the whole world - and furthermore, will take that massive world of yours and reduce it to a comprehensible size. If you'll pardon me taking my metaphor a bit too far, it's like looking at one cell of your plant, and realizing that it's got DNA in there, that if you could look at it the right way, you'd be able to learn some things about what the plant was like. A character from one section of your world won't know every detail of every little corner of that world. But if you've built the connections well, then the view from the spot where that character lives will give tons of hints about the existence of a larger structure - the bigger, more meaningful entity that is their world.
There's a reason why I love to use first person point of view, and third person internal point of view. When you have a world that big, it's hard to manage it all, and keep its information from overwhelming what the story is about. It's the wonderful myopia of a culturally situated character that allows you to cut it down in a way that makes sense.
So start with a story seed, then grow your tree with as much care as you think necessary for the story's needs, and finally identify the single part, the character, who will allow you to create the most meaningful view of the whole. Maybe it shouldn't be world building at all, but world growing.
At least, that's how it feels for me.