After clicking a choice, you’ll see a placeholder that says, “Add Headings (Format > Paragraph styles) and they will appear in your table of contents.”
5. Compose your doc, adding headings throughout.
6. To add headings, hit the “Normal text” drop-down on your toolbar, select either “Heading 1,” “Heading 2” or “Heading 3,” then write out the headlines for your document. These headings will become the labels on your eventual table of contents.
Original Source: https://blog.google/products/docs/how-to-google-docs-table-of-contents/