You load the first chapter like any other one-shot. Then, on your account page, go to "Stories", find the one you want to add a chapter to, and then click "edit" .
Near the top of the next page, between "Properties" and "Live Preview", you get the "Content/Chapters" option. Click on that, and one of the options you'll get is to upload a chapter to said story using anything in your Documents.
I hope this was clear enough, but if not, just let me know!
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