Actually, the point of a disclaimer when writing a fan fiction of a published work is to establish clearly that you are not attempting to infringe on the copyright of that material. You're not claiming the authorship of the fandom, or its unique characteristics, or its canon characters, nor are you making any profit from your fan fiction.
Should a published author decide they don't want fan fictions of their work out there, they have pretty clear cut avenues to follow. A published author will file a cease and desist order requiring the archive to remove ALL fan fictions of their works, or will have their legal people contact an archive looking for contact information and identification of the author(s) who've used their intellectual property.
Now, when you turn your sights to an original work published here on AFF, you're entering a new arena. Original works published here are presumed to be the copyrighted property of the author. However, unlike a well known fandom, you need to actually have permission in writing prior to embarking on your fan fiction. Otherwise, it is impossible for you to defend yourself against an accusation of plagiarism from the author of the original work, should they take umbrage. And without any written permission from the author allowing you to use their work, we will wind up deleting your fan fiction, and your account. We are adamantly opposed to plagiarism.
As far as the actual mechanics, it's the same thing you've done in high school, in college, in university. You cite your source. We don't require a specific format, but what we do want is the author and title of the work used, and where applicable, a page where the quote can be found.