Take a tiny seed of an idea, from a book or story title, maybe from a sentence in a story (yours or someone elses, it doesn't matter.) It can be a single word or image or phrase. Begin with “Once upon a time” and just write. Do not be concerned about language or style or spelling. Just free write. You never know, something might develop.
The Phantom (from The Phantom of the Opera the CD of which just happens to be sitting next to me on the desk.)
Once upon a time there was a phantom. And a most wispy, ethereal and mysterious phantom he (or maybe she) was too.
(I'll say this phantom was a she, simply because so many phantoms seem to be of the masculine gender.)
This wispy, ethereal and mysterious phantom whispered and moaned about a lonely cottage that was perched high on a lonely hillside. A single tree grew next to the cottage and the phantom used to get her wispy trailings caught in its branches which irritated her to no end. But what could she do? She was a phantom attached to the cottage, doomed to whisper and moan about its door and every once in a while go sweeping upwards to roof, wailing loudly. It was usually at this point that she'd forget herself, forget her long wispy trailings and invariably get tangled in the tree.
Etc. etc. etc.