How do you use fable to make a sentence?

Start with your moral. A fable has to be written "backwards" because you need to know the end before you start writing. Then, just think of a way to explain that moral by telling a short story. Remember the examples of fables: the lion helped by the mouse, the fox with the grapes, or the dog who saw his reflection in the river. Your story should be along those lines -- show the reader about your moral instead of just saying the moral right off.
First, you have to have a moral. Then, you have to write about an animal.