This article will show you how to convert a normal heading to a checklist item when working on a checklist article. This can only be done in the desktop editor.
Verify that the article type is Checklist
You can only add checklist items to articles of type Checklist. In the Article Inspector verify that the type is set to Checklist.
Locate the heading you would like to convert
Headings that are already checklist items have a checkbox icon next to them (1). Standard headings will not (2).
Right-click and select Convert to Checklist Item
Right-click on the heading and select Convert to Checklist Item from the contextual menu.
The heading will now be a checklist item.