Once you're signed in on Joomla (you should be automatically if you're logged in on the forums, but all the same, check the top left), go to the main page of fanfic, and there's a button in the middle that says "add story." Click it, and fill out the forms there.

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If the world is to be destroyed, so be it. If my fate is to die, I must simply laugh.
Read up on the Rules!