Here’s what you have to do:
1. Read examples.
2. Write a brief synopsis of your novel.
3. Write a briefer synopsis of your novel.
4. Think about what the core question that you MC is trying to answer, the root of the entire novel.
5. Write that down.
6. Write it down another way.
7. Write it down even another way.
8. Now do your 99 words. it can be longer than 99 words, but get down a draft. The trick with flash fiction is you want you reader to experience the same things they would in a longer piece–a strong MC, tension, a tight plot, etc. etc.
9. The key is going to pick the right scenes. In order to get your readers to feel, you are going to have to some showing (rather than all telling). Think hard about where you’re going to slow down. What pieces of dialogue are you going to use and why? Every word should count. Every. Single. Word.
10. Get a draft out as fast as you can and then spend the rest of your time revising. Don’t be afraid to start over. After all, it’s only 99 words. Revise revise revise and then get some family members to read it for you. See if they connect to the MC. See if they laugh. See if they are surprised. Then revise, revise, revise. I love this challenge!
Most of all, have fun!