Write down a brief character sketch of the main characters in your story. Write these character sketches in a notebook. Include the name of each character, a brief physical description of each character and the character's role in your story.
Outline your story. Fifteen pages isn't a long short story, so the outline won't need to be as detailed as an outline for a novel. A simple outline to keep you on track will be sufficient. Think beginning, middle and end. Write down a brief synopsis of how your story begins, what complicates the story for your main character in the middle, and how the story ends. That will be enough of an outline to keep you on track as you write.
Set a daily word goal. Five hundred words a day is not a lot of words. If you write 500 words each day, without fail, and follow your outline, you will have the first draft of your 15-page story completed.
Read through the draft and look for dialogue you might want to polish. Look for errors in the plot or events that you might want to rewrite. The rewrite stage is easy. The challenge of the first draft is gone. If it helps, put the story away for a couple of days and come back to it with fresh eyes before you do the final polish. Setting and meeting these goals can help you complete your 15-page story, or any other writing projects you have.