Yesterday, I posted about the short story publication process.
Now, let's talk a little bit about how you write to a theme, for, say, an anthology or a theme-heavy magazine/contest.
It doesn't start with the writing process--it starts with the planning. You need to latch onto something for inspiration. Now, sometimes you can get lucky and snatch onto something straight away, but it's not necessarily the norm. So, find yourself a menial or repetitive task (ask my neighbors. I walk circles around my property almost every day. I do it so much I know my path, riddled with rocks and junk cars as it is, well enough to walk it in the dark.) and have at it. Go over whatever you need to, and give yourself at least half an hour of uninterrupted thinking time. If you have a longish commute to work, that's the perfect bloody time to go about it, too, you lucky you.
Of course, that's no guarantee to you figuring out your idea. Don't sweat it. I once went over half a year sitting around saying that an idea just wasn't worth writing for, and then, in the middle of NaNoWriMo, three days before the deadline for the antho, I got hit with the bug and had to write for it, because I got the idea. Inspiration, you see, isn't just finicky and fickle, it also has a tendency to show up at inopportune moments.
Of course, that's also the best time to grab it, when it sneaks up on you all kinds of ninja-like.
It may be full-fledged, it may be an inkling, but now you need to nurse it. However you do it--brainstorm, list make, draw it out, map, bubble-chart--don't leave yourself with some barely formed idea, even though it is brilliant. This is where you get to flex your writerly muscles, especially when it comes to speculative fiction. Worldbuild.
Whether your idea was for a character, a plot, or a world, worldbuilding is pretty much your first step, or one of your first steps. You don't have to go overboard, just enough to grab an editor's attention.
Now, check it again here, right before you set out to write it. Does it still actually fit the theme? If it does, awesome. If it doesn't, awesome. Write it no matter what, because you can always track down some sort of home for it, it might just take a little more searching.
Now, we go to the same process as any other publication run, with one preliminary step: does it work for your theme? After you've written it and let it breathe, edited it and refined it to it's publishable form, is it still good for your original theme. If so, and you don't think it belongs somewhere else all of a sudden (sometimes it happens, and you just have to play around at Clarkesworld or Writers of the Future instead), throw it at the intended market.
It's the same thing, with a little more emphasis, and a little more constriction, put on your idea sessions. Consider it a workout--it's more weight on the arm-lifty thingy machine thing, and it will make you a better writer.