Choose the type of horror story. You can choose to write a psychological horror story or a traditional scary story that is full of surprises. Your story can feature maniacal killers, inexplicable events, a variety of supernatural entities and even the subtle horrors of everyday life. Psychological horror stories often affect the reader deeply mentally. Traditional scary stories typically move at a faster pace.
Outline the characters. You don't need to jot down secondary characters, but you need a rough outline of the main characters. Define the personality of each character. A good way to do that is to do a character interview. Ask yourself questions about each character and then answer them the way that character would. For example, each character should have different fears, interests and faults to make a rounded personality.
Brainstorm a plot. The plot of a scary short story has to carry the story, so it needs to be engaging, thought-provoking and frightening. An example of an aforementioned plot would be: a young man wishes to see his girlfriend before she leaves to study abroad, but his car suddenly dives into a ditch, leaving him at the hands of mutants.
Create a setting. A scary setting needs to evoke a frightening feeling. For example, if you use the above plot, you could create a setting where the main character is enclosed in a tiny house that's dimly lit, with smell of rotten sewage emanating from the floor.
Choose a point of view. For short stories, you need to choose one point of view and stick with it. You can choose from first person, second person or third person. Second person address the reader directly as "you" and is rarely used in stories. First person allows the reader to connect with the character better, as everything is written from the perspective of the lead character and includes only information that character could know. Third person allows for better story development and provides for a narrator that can describe more for the reader than the characters know directly. First person works well with psychological stories.
Write short sentences to create tension. Scary stories are full of tension; you need to keep the reader wanting to know what's going to happen next. The best way to accomplish that is to shorten your sentences. For example, instead of writing, "A loud thud reverberated through the room; it looked like something was nearing John," you could write, "A loud thud reverberated through the room. A figure approached."
Wrap it up within 10,000 words. While some short stories have gone beyond 10,000 words, most come to an end at around or before the 10,000-word mark. Some professional writers' organizations, such as the Science Fiction and Fantasy Writers of America, cap the maximum word count for a short story at 7,500 words. Your ending should tie up the loose ends and resolve the story.