How do I create a section that is not a chapter (Sub-Section)?

You have to start with at least one chapter, then scroll to the bottom right-hand of the editor and click on the "Add New Block" button. This will take you to the Table of Contents, in which you will be able to select "Section" in the drop down list. Lastly, you will have to enter in a section title.

You can also turn a chapter into a section by dragging and dropping it on top of another chapter in the chapter menu.