While this is not mandatory, it is generally recommended to add a new course category to organization the courses that represent series.
Step-by-step Guide
Go to Site Administration > Courses > Manage Courses and Categories.
Click on “Create New Category”
Set the “Parent Category” field to “Top”
Use “Series” for the “Category name”
Click on “Create category” to save your new category
Your new category will appear at the bottom. You may use the “Move up” arrow to move it to the top or anywhere you would like.