To insert a footnote in a Google Document, at the end of a needed source click on Insert and scroll down to Footnote.
A text box will appear at the bottom of the Google Document. Type the proper source citation or extra content needed for the footnote. When you are done writing the footnote, click outside the box and continue working in the main document. If you need to go back in the same footnote, click that footnote to continue typing.
To see how your footnotes will appear when printed, go to Print Preview from the File Menu. You can also view your footnotes in PDF by going to the File menu.
If needed, for whatever reason, you can delete any footnote. To delete, click on the footnote in the body of the article (the little number that appears next to the text), and delete the number. This will remove the footnote from the document.
Unfortunately, Google Drive does not have a feature that allows you to create end notes.