Your story saves as a draft while you write, so you can close it and come back later on any device. Drafts are private: nobody else can read a story until you post it.
Find your drafts in My Stories under Not posted yet. Tap one to keep writing, or tap Post when it's ready.
To post, add a title and at least one line, then tap Post Story. Your email needs to be verified first. Save and Finish Later keeps it as a draft and takes you back.