To create an output file for your story that is readable in a Web browser, choose Build Story from your story map window's Story menu. This prompts you to choose where to save your finished story; make sure to give the file a .html file extension. After you've saved your output file, Twine will open it for you in your Web browser.
Once you've build your story once, you can quickly rebuild it to reflect changes you've made by choosing Rebuild Story from the Story menu. For ease of access, this menu item also appears in the Passage menu of passage editing windows. Keep in mind that you'll need to reload your output story in your Web browser once you rebuild it. If you lose track of the file, the View Last Build menu item will open it your browser once more.
To check how long your story has become, choose Story Statistics from the Story menu. This counts how many words, passages, and links your story contains. It also displays how many broken links there are, so you can do a final check to make sure your story is complete.
You can generate an RTF version of your story suitable for proofreading by choosing Export Proofing Copy from the File menu. This compiles all of your passages into a single document that can be opened up in a word processor like Word or LibreOffice. Note that the Proofing Copy is very limited in which Twine syntax it can contain: many syntax structures, like images, HTML, macros and whatnot, will be left as they are.