Brainstorm morals or lessons that you want to write about -- e.g., slow and steady wins the race, always trust your heart or never overeat. Select a single moral that the story will convey.
Choose two characters to represent either side of the moral debate. Use animals who can be personified or well-known characters who can be represented as foils -- e.g., the ant is known to be community-minded and an industrious worker.
Create a situation in which the characters would have to disagree or express their moral differences through action -- e,.g., in "The Tortoise and the Hare," the tortoise and hare argue about their differing perspectives and decide to settle the disagreement by competing in a race.
Write an outline of the plot's events, starting with the introduction of the characters and ending with the conclusion of the plot or a statement of the moral. Use the outline as a guide for how you would like your fable to flow.
Write out each event.; you can ignore the order of events and write as you are inspired if it makes it easier for you. Try narrating the story and recording the narration to start developing the narrative.
Edit your story thoroughly and revise as needed. Add events and sentences to improve the transition from one idea or scene to another. Edit grammatical and spelling errors.
Share your story with others and encourage feedback/criticism. This will only help your story improve if you are open to suggestions.