The trick - allegedly - to writing short stories is the ability to find your writer's voice. This is the one that naturally comes forth when you allow yourself to romp through words without thought, i.e. free writing.
If you think about how different everyone looks physically then it stands to reason that we each have our own distinctive writing voice. For example:
'When Mr Bilbo Baggins of Bag End announced that he would shortly be celebrating his eleventy-first birthday with a party of special magnificence, there was much talk and excitement in Hobbiton. Bilbo was very rich and very peculiar, and had been the wonder of the Shire for sixty years, ever since his remarkable disappearance and unexpected return.' The Lord of The Rings, Tolkien
'Watch. . .
This is space. It's sometimes called the final frontier.
(Except that of course you can't have a final frontier, because there'd be nothing for it to be a frontier to, but as frontiers go, it's pretty penultimate . . . )
And against the wach of stars a nebula hangs, vast and black, one red giant gleaming like the madness of gods . . .' Moving Pictures, Terry Pratchett
'Wind howled through the night, carrying a scent that would change the world. A tall Shade lifted his head and sniffed the air. He looked human except for his crimson hair and maroon eyes.
He blinked in surprise. The message had been correct: they were here, Or was it a trap? He weighed the odds, then said icily, "Spread out; hide behind trees and bushes. Stop whoever is coming . . . or die." ' Eragon, Christopher Paolini
My bookcase may or may not be skewered to a particular genre. But the point is that each book sounds different. And so to the elusive short stories - where do they come from? Conventionally they can follow guidelines such as conversations on a bus, what if..., seeing someone in the street and telling their story, your own emotions about an event and indeed your past experiences. But what if you allowed your writing voice to dictate your short stories.
Without thinking write down three story titles.
Who told you those? Where did they come from? A blend of your own imagination and external influences from books you've read or films you've watched perhaps - but what if there are a queue of characters in your head just waiting for you to let them tell you their story.
Now take one of the titles and allow it's main character to tell you their name. Start writing My name is ... and then describe them. Try not to write story, just write about the character. Don't think, just write - as soon as you start thinking, stop!
Finally take your story title and your character and write a few sentences telling you what the story is about e.g. The Bandstand is a story about Geoff Brider who, after his wife walked out and he lost his job, tries to save a derelict band stand in his local park. Geoff finds himself swept along by batty park defender Margot and together their unlikely alliance develops into friendship.
Et voila. Short story beginnings. The big question, of course, is how long is a short story. Who knows? But let's address convention again - Flash Fiction you're looking at about a 1000 words - short stories are generally 500-3500 - novellas 18,000-35,000. In the words of Ian Ayris short stories are the jogging you have to do before running a marathon that is writing a book.
The Ray Bradbury challenge - over the weekend free write as many titles as you can without thinking about it too much. On Monday pick one and write the story. Spend the rest of the week reviewing and refining. Repeat each week until you have a short story collection. Send them out to magazines and competitions, create that ebook, gift them to friends and family. Imagine the writing experience you will gain.