Footnotes may be created in the editor by placing the cursor where you would like the footnote link, right-clicking, and choosing Insert > Footnote. This will create a linked number to the footnote item, like the one following this sentence.
You may edit the visible text of the footnote either in the Ribbon or by editing the element directly.
When you add or remove footnotes, the footnote list should be updated appropriately. If not, simply click on an existing footnote, and it will update the numbers appropriately. Footnote numbers will also be updated when the page saves.3
Note: If you delete the footnote link / reference, then the footnote text will also be removed when the list is updated.
If you would like to manually style a block of text as a footnote, you can also right-click on a paragraph and choose Formatting > Footnote.