For creating an outline, the SERP Explorer is a useful option.
If you click the "Outline" tab and "Open SERP Explorer", you will see a horizontal view of the SERP results pulled for the query you entered to create the document initially.
You can filter by typing in the search bar ("filter by keyword") at the top of the window, and can choose H2, H3, ect. from the "Heading Type" dropdown.
Scroll horizontally to look through the sources, and click any of the headers on this view and they will be highlighted in green, and added to the view on your right. Also in this view, click "+add custom header" to type out your own header and set the header tag.
You can click the small settings icon on the corner of a header in the right-side panel to change the heading tag, and also edit the text of the header itself. Click the green checkmark to save any changes.
When done compiling headers, click the green "Save Outline" button.