Well, firstly go to the Showcase section and check out how other stories are published.
People usually like to read a really well told story about the background of your custom mod, and see some pictures of the maps, rooms, enviroments so they can realize what's it all about. A video would be a really nice addition. (To create the thread click on "New Thread")
So basically it should be like:
A Morbid New Year
[BACKGROUND] So the story begins in Febuary 2nd, 1749. Daniel has prepared for the .... blah blah
[PICTURE 1]
[picture description]
[PICTURE 2]
[picture description]
More pictures
Video
Download link
So basically your thread will look nice and readable, and will pass the basic background of the story which will help people decide wheter they want to play it or not.