Not that it matters, but I thought I'd mention that I don't work that way. I mean, I think that's what real science-fiction writers do, but it isn't the way I look at things.
What happens is more like this: Some culture (or, more likely, some facet of some culture) sets off my "Oh, that's cool" alert. Then I set out to learn about it as it actually is--not looking particularly for, "can I believably make this happen?" but just trying to understand it as well as possibly can before my patience runs out. Real science-fiction writers have more patience than I do, but that's neither here nor there.
Then, once I have it in my head, I start using and abusing it however the story requires.
I remember once Harry Harrison took me to task for putting maize in a setting that corresponded to a place and and a time where maize didn't exist (Brokedown Palace, to be precise). To me, the whole point of fantasy is to be able to say, "I think I'm going to create late feudal or early Renaissance Hungary, only stick maize in it so they can eat all of these kinds of things.
My point? I don't have one, really; I just thought some of you might be curious about how I approach this stuff.