Well, start with an interesting hook. It could be a strange dialogue or an unexpected situation. Then, keep the plot simple yet engaging. You don't have much space, so avoid complex sub - plots. Also, use strong imagery. Instead of saying 'it was a dark night', say 'the night swallowed everything in its inky blackness'. This makes the story more vivid in just a few words.
First, have a clear concept. Decide on a single, strong idea quickly. For example, it could be a unique emotion like sudden regret. Second, create vivid characters with just a few strokes. Maybe describe a character's nervous habit, like constantly tapping a finger. Third, make every word count. Cut out unnecessary adjectives and adverbs. Don't say 'the big, red, shiny apple' but 'the shiny apple' if the color and size don't matter to the story.