Okay so, firt anda have to write it up on your computer EG: Notebook, Wordpad etc.
Then anda go log into your profil (On fanfiction) and go to Publish: 'Upload document' anda then upload it and go to new story, pick your characters, rating, judul etc and upload it on there and either preview, ubah atau both.
(be sure to ubah it just incase)
It could take awhile to appear on the site but it never takes me long so anda shoould be alright :D
I think that's how it goes I havn't done it for a little while :)