Have you ever watched a show or a movie and wished the story or characters had gone another way? Or read a story or book and thought about the characters afterward, or imagined what the story would have been like if one of the secondary or minor characters had been at the center?
Yes, I know, that's called fanfic or fan fiction, and many writers frown upon it. My point is, it's a form of writing to a market, and it's a start.
What you want here is the ability to analyze a magazine or anthology to see what kind of stories they like, and if they're the kind of stories you have in you. Some people go so far as to outline printed stories and write character sheets on the characters. After they've done this for several stories or book titles from one place, they know what appeals to the editor. Others just read the stories and see if they get a feel for common elements.
For instance: Do the stories all start with the conflict apparent in the first few paragraphs? Does it matter if there's a lot of description of setting or character? How much interior dialog? Are the characters' feelings and relationships explored, or is it get 'er done
and keep it on the surface? Are cats okay? Talking cats?
With these common elements in mind, maybe you can build a story incorporating them. Or maybe one of the stories that just come to you will magically have these elements as part of it, just because you've put that set of blocks in your toybox.
I'm all about writing a story that's true to itself, and faking something up to try to fit a market may or may not work. But growing something suitable is not the same as faking something up, any more than growing cabbage to make slaw is the same thing as shredding
broccoli stems and calling it slaw. Maybe you like broccoli slaw, but it still ain't cabbage.
I think this is another metaphor that got away from me....