Navigate to the page you wish to modify

Click on Edit at the top right of the page.

Click on the + sign located on the left hand side of the page to add a new section.

In the Section layout gallery select a layout.

The new section will appear.

Ensuring the new section is selected click on the Edit Section icon in the left hand bar..

the Section panel will appear on the right of the window.

Move the Make this section collapsible slider to On. 

Options will appear allowing you to tailor your collapsible section.

You can elect to Show a divide line between sections and the icon alignment.

You can elect to make collapse the Default diplay.

Click on the cross  in the upper right to close the edit panel.