You can do it over 12 consecutive days, 12 weeks, every other day for 24 days, whatever pattern works for you, though it's always a good idea to set your writing time in advance. You'll need half-an-hour to an hour for each prompt. Don't worry that other people are using the same premise: one idea in two different writers' hands becomes two very, very different stories. (Besides, the premise is very wide open!) If you want, though, you can always pick a premise or story idea from your own collection of ideas and use the same prompts on that premise.
Bookmark this page and then each day click through to the next prompt, from the list below.
Serving suggestion: Get yourself a notebook specially for this project and write by hand. When you need to cut and glue things, you can paste them into your notebook. It becomes a wonderful little story scrapbook and it's just fun to have physical things sometimes, instead of working at the computer.
Motivation suggestion: Want to keep yourself going? Arrange with a friend that you're both going to do it, and then message each other before and after each writing session, to say "I'm going to start now" and "I've just done it". You're also welcome to email me with your progress: email@example.com or tweet me @WritersGreenHse. I'd love to hear how you get on.