Creating a series? If it's on an existing story, find that story and click on "edit." For a new story, you're already at the right place.
Scroll down the page to the "Series" box and write the name of the series. Click on "create" the series when the option pops up, then hit publish.
If you need to reorder the series, go to your stories, choose the series option, and edit.
Have fun!