A footnote reference is simply a number in square brackets. To define the footnote, place this definition at the bottom of your file. ‘footnote-mode’ can be used to greatly facilitate the creation of these kinds of footnotes.
Footnotes are defined by the same number in brackets occurring at the beginning of a line. Use footnote-mode's C-c ! a command, to very easily insert footnotes while typing. Use C-x C-x to return to the point of insertion.