Ok, just follow these steps. :)
1.) Post your story to your journal.
When posting your fic, please use this form(or something close to this) in the story info space:
Note: This is a Supernatural Slash Fiction Angstathon Entry. For the ficathon, I wrote for (LJ username), who wanted (list their fic preferences).
You don't have follow the form exactly, but just make we know who you wrote for and what they wanted.
2.) Fill out this form as a comment to this post:
I wrote for:
Rating & Warnings:
Main type of Angst:
And You're done. :D I'll be posting a speculative list tomorrow. We did have a lot of people write in with late fic notices, so fair warning.