In SmartCue, sections or chapters are referred to as 'Highlight Modals'. Here's how you can add one to your demo:
Open the Builder for your demo.
Locate the Highlight after which you want to add a new section.
Click on the plus (+) icon next to the Highlight.
Select 'Highlight Modal' from the drop-down menu.
Add a suitable Title and Description to your new section.
And that's it! Your new section or chapter is now part of your demo.