Every writer works differently. The only real magic in the process is finding what works for you. Some writers need to know what they're writing toward. Others don't. In an interview for my blog, Letterress, Stacey Richter, author of the short story collections Twin Study and My Date with Satan, says she has some elements in mind when starting a short story, but not all:
I know where it's heading but I don't know where it will end up. I always try to start a story with some sort of conflict in mind. That usually gives me enough direction to write the beginning and the middle and at some point after that I have to figure out how to end it. Occasionally I try to start a story with only an image or a world or a character I like. Sometimes that works too, but I've found that those stories are more likely to get bogged down and abandoned.
In an interview in Gotham Writers Workshop's short story anthology Fiction Gallery, Hannah Tinti, author of the story collection Animal Crackers and the novel The Good Thief, says she doesn't plan her stories out ahead of time:
I've never been much of an outliner. Instead I write very, very slowly.
Of her story “Home Sweet Home," which begins with the murder of Pat and Clyde, she says:
I started with describing the murder, so in a way the plot came first, but I had no idea who had killed Pat and Clyde until I went into each of the characters . . . These details explained the characters' motivations, giving me a better idea what the action might be. Once I solved the crime and realized who the murderer was, I added some finer points to make it more believable, but since the characters had led me there, I didn't have to alter the plot.
Whether you like to have it all mapped out ahead of time or prefer to launch into a story with just a snippet of dialogue or a shimmering image you can't shake, be aware that the process of writing is an act of discovery. As you sink into individual characterizations and specific scenes, you will learn more about your story and your intentions. Writing the story allows you to experience it and this brings to the surface details and understandings you couldn't possibly anticipate.
Writers who plan need to remember to stay open to possibilities during this process. You may discover something that wrenches you away from your initial ideas or you may stay close to your plan and go deeper in a direction you didn't expect. Writers who don't plan need to stay attuned to discovery, too, as a way to sharpen and focus the story.
Don't expect that what worked for one story will work for all of them. Over time, you may find you have an approach that usually works for you, but a story idea may come along and call that all into question. If you avoid outlines and a story's shape and momentum simply slips out of your grasp each time you try to write it, a little planning might be just what you need to fix a few elements in place long enough to finish that first draft.