Well, I may not be the best at explaining it but I can sure as hell try! :')
SO! You can click on your gallery, and select 'submit art'. After that it takes you to a page with a blue box, and 'enter text' should be there! Click on that, then copy and paste your story into the box that pops up! Beware though, if you want italics or bold, you have to use <.i.> and <./.i.> to do it (but without the full stops!) If you want bold, replace the 'i' with a 'b' and you've got it! ^^ So it does this and also this! But you may already know that, just throwing it in there! :')
Anyway, after that click 'preview' just to check everything, then done when you're happy with it! Then all that's left is to title it, and put a little message in the box under the title if you wish! Oh and you have to say if it has mature content or not! That's 100% MUST DO. You can also add an image if you like by clicking the little pencil in the corner where your fanfic writing is, click 'edit preview image' and you can pick one from the saved ones on your computer! (Or on your phone like I do xD)
After that you pick a genre, which basically shows it's 'Literature', then you have sub categories like 'humour' or 'romance' or 'teen-fic' - just pick what you think is best! ^^
Then scrolly down to the bottom and click the green button which says submit! ^^ And there you have it!
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I am so sorry this sucked -.-