I'm four days late, but I'll give you a hand. I've got links to pictures below each step so you can see what I'm talking about.
Step 1: Find your story and click the the pencil icon (edit).
Step 2: Next to "Edit Story" should be "Add Chapter"
Step 3: name the chapter and enter the text in the big white box. You'll only have space for 9,000 characters, so select your text in bites.