If you'd like to post a series work, you'll need to select "Series" in the "Posting format" section of the novel posting page.
This will let you select that series, so you'll be able to add new works to it. You can also add a new series.
If you'd like to post the second episode of your series, you'll need to use the dropdown menu to select the series name to which you want to add works. By selecting the series and then posting your work, you'll be able to see each work page within the series linked in the upper right. If you'd like to post a single entry story, please select "One-shot" when posting it.