Establish your genre. Make a list of labels that could apply to your short story. Broad categorization with labels, such as fantasy, science fiction or literary fiction, can be simple. Narrowing the label down to slipstream or magical realism, however, can be more difficult.
Research possible markets within your genre. Start with the broadest of categories -- lists of markets obtained through a database site, such as Duotrope's Digest or the list of markets published in yearly anthologies, such as "The Year's Best Science Fiction," or "The Best American Short Stories."
Narrow down your list. Many magazines, printed online or off, offer samples or free content on their official websites. Most print journals also offer sample copies of their magazines as a cheaper alternative to purchasing a subscription. Read as many excerpts or full stories as you can; this will help you find the markets that are the best match for your fiction.
Look for themed issues where applicable. Duotrope's Digest maintains a list of upcoming deadlines for themed issues and contests.
Begin with a very small list of possible markets, narrowed down from your earlier choices. If your selected magazines accept simultaneous submissions -- allowing writers to submit the same story to several markets at once -- then send your story to three to five markets at once. If your selected markets do not allow simultaneous submissions, and many science fiction and fantasy markets do not, then send your submission to only one market at a time. Always check the magazine's guidelines.