Through the series management page, you can create a new series, add a description for it, and add existing stories to it. You can also delete existing empty series.
Once a series is created, it will appear in the submission wizard when you're submitting a new story. You can select to link or not to link the submitted story to a series. If you choose to link it, then the story will appear on the site with a link to the series in its description.
You can always add stories to a series or remove stories from it.
Each story in the series can have an optional little blurb, separate from the story's description, that describes how the story fits within the series.
Do not confuse 'Series' with 'Serials'. If you have a story that you want to post in installments on the site, then don't create a series for that. Series are comprised of whole stories or serials. A serial can belong to a series, but a serial does not make a series.
Do NOT create a series for just one story.