Think of a small but memorable incident that has happened to you, perhaps in your childhood. If I was doing this exercise I'd write about the rabbit that ran into our garage when I was about eight, and how we ended up adopting it, but it bullied everyone - the cat ran away, the hamster died, and it wouldn't let us watch television - so my father decided The Rabbit Had To Go...
Now tear up some pieces of paper - A4 to four pieces perhaps - or you could use index cards or sheets from a notebook. You want to end up with about 16 or bits. Write out the story using one, perhaps two sentences per bit. So my story would go something like...
Piece 1 - My brother said there was a rabbit in our garage
Piece 2 - My mother caught it
Piece 3 - No one knew whose it was, so we adopted the rabbit
If you need more bits of paper it doesn't matter, if it fits onto only 8 pieces, then that's fine too. When you've done that, go back and add some details, some descriptions of people or places. If you run out of room, re-write on a new bit of paper Try to add some details that fix it in time and space so...
I was on the sofa watching Crackerjack when my little brother came in and said there was a rabbit in our garage.
Crackerjack sets it in the 60s/70s (and for those who remember, specifically: it's Friday, it's Five to Five and it's......Crackerjack!).
Done that? Now, look for places where you can add some direct speech.
'Quick, quick, come quickly!' my little brother said, rushing into the room. I looked up from the sofa where I'd been curled up watching Crackerjack to see him hopping from one knobbly knee'd leg to the other. 'There's a rabbit in our garage!'
When you've done all that, write it out as a continuous piece of prose.
When we write we normally think of the details and let the story take care of itself. This exercise pushes you to making the story move forward, because the initial stage is about nothing but the essential story line. So we start with the forward movement of the story, and the details get added as required, and then finally the dialogue comes. What you should end up with is a great little story with some sparkling detail, lively dialogue and a cracking pace.