So first you want to start with the hidden story. What’s the message you want to get across, figure out what your topic is and what you want to say about it. Second you have to break the hidden story down, what are the most important characters or aspect of the story. These will become the main characters, settings and events in your allegory. Third you have to pick a theme for the surface of the story and find correlations. The surface story should be different from the hidden story. Science fiction and fantasy work well.
Hi. Your question is incomplete because you didn't show what should be used in the essay. This prevents the essay from being done exactly as you need it. I hope this helps you and that you can adapt it to the requirements of your question.
