In the top right corner, put your mouse over your name, click on the link that says 'My Works' Find your story and on the right of it there should be something that says 'add new part'. I hoped I answered your question sufficiently :)
Ignore User
Both you and this user will be prevented from:
Messaging each other
Commenting on each other's stories
Dedicating stories to each other
Following and tagging each other
Note: You will still be able to view each other's stories.