Go to your class site and click "Edit settings" in the Class administration block
Scroll down the Edit Class Settings page and expand the Course Format area
In the Format dropdown, select Collapsed Topics
The page you are on will now expand and you will have several different options to control the look of your collapsed topics and how they work for the user. Each of these topics has a short explanation if you click on the ? icon beside it. There is also a link on this page for the full readme on the collapsed topics plugin.
Click Save and Display at the bottom of the Edit Class Settings page
You will see that your topics are now in a collapsible format with the settings you chose