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