If you are starting from a text file, I would expect that having "Chapter" whatever on a separate line would be enough. That is with the default chapter detection on the "Structure Detection" page. If that isn't working, or you are using something different to mark your chapters, we need to see an example to decide what is needed in the conversion process.
But...
As it is fanfiction, have you looked at the
FanFictionDownLoader plugin? That will download fanfiction from a lot of different sites and create the book for you.