Find a published anthology and write a story for it.
You’re basically pretending an editor has contacted you and requested you write a story for the book. What I like to do is head over to Amazon.com and search specifically for anthologies. I’ll pick one and use the anthology description as my starting point.
Now you might be thinking, hold on a minute, if the anthology is published, then what chance do I have of getting into it? Well, of course you can’t get into that book, but trust me, there’s lots of other potential markets for your story. I find that this idea actually works best when I haven’t read the stories in the anthology — I’m less likely to be influenced by the other writers — but I certainly encourage you to buy the book when you’re finished. And it might be fun to see how the other writers approached the same idea differently.