Brainstorm for a few moments. Think about what sort of story you want to write: mystery, romance, horror, metaphorical, science-fiction tale or other kind. Imagine an interesting plot-line. A short story often reveals a plot that takes place over a few hours or minutes. However, a short story can also tell of a plot that takes place over many years, as it describes the actions of the story over time by looking back or summarizing. Make some mental notes about plot ideas and general settings. Think of the time period, location (city, country, outer-space) and setting (house, garden, various memories of settings). Think of what the general story is about, any problems that may arise and the story's resolution or ending.
Identify your characters. Writing a short story quickly may not allow for the introduction for many characters, so think of one or two that will guide the story. Think of who they are as people, what they want from the world, where they came from, and what they're doing in the story. Make each character distinctive so that they can be identified by the reader. According to author Anne Lamott's instructional writing book "Bird by Bird," plots are often carried by characters. An otherwise simple or bland storyline can be made interesting with a character whose personality is defined clearly. Readers connect with characters when they are intrigued by or sympathetic to their behaviors. If a character is designed to want or feel certain things, it can have an impact on the what direction the plot takes.
Write the story quickly by creating an opening setting, a possible conflict or point of interest and a resolution. Do not rehash plot points in your head; follow your instincts for storytelling in the moment so that you can complete the story quickly. You can open with a description of the setting or the character's thought.
Continue writing. Introduce your characters and reveal a major plot point. Write about how the characters react to this plot movement. See where this takes you. Rely on your instinct and understanding of your character to develop the plot as it goes. Keep in mind your setting, location and character qualities so that the story is consistent. Do not mention in the beginning that it is night only to later mention "the sun was shining."