For creating an outline, the SERP Explorer is a useful option.
If you click the "Outline" tab and "Select Headings from Search Results" button, 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 red "x" icon to delete any headers. You can drag and drop the headers by clicking, holding and vertically moving them with your cursor. And you can also change the header tags by clicking, holding and swiping the header quickly to the right (to move up to an H5) or quickly to the left (to move back to an H1).
When done compiling headers, click the green "Save Outline" button.