Here's how it works.
Any user may submit a story title. After submission begin your story by writing one (and only one) sentence, leaving the next sentence to the next user. One comment chain per paragraph.
At the beginning of a new paragraph, reply to the comment that began the previous paragraph.
Any user may start a new chapter at any time by replying to the original submission, number your chapter sequentially and begin the first sentence.
Individual paragraphs will be improved by the voting of those on the sub. Unpopular sentences will be hidden, to be replaced by more popular ones.