I've updated the spreadsheet prepared last year by a completely spectacular set of volunteers, removing prompts that were written last year and adding in this year's prompts. I'm pretty sure I got them all, but if you can't find a prompt you left, please let me know.
To claim a prompt, find one you want and then leave a comment in the spreadsheet--something along the lines of "Claimed by"--that includes a username you use either here or at AO3.
If you have trouble leaving comments there, you can comment here with your prompt instead. Please copy and paste it exactly as it appears on the spreadsheet, and please include your AO3 username if you don't have an LJ or DW account.
But before you begin any prompt claiming, I recommend:
1. Reading the rules.
2. Taking a moment to look through the spreadsheet, which is well organized but rather massive. In particular, you may want to look under the first tab, "Explanation," and scroll down to "Our categories" to see all the existing tabs. (Note that not all of the tabs will be visible without clicking the arrows.)
Prompt claiming is first come, first served. Please check that no one else has claimed your prompt before requesting one.
You can find the spreadsheet here.