Determine the footnote style. Each style guide, such as Chicago Manual of Style or MLA, contain a different set of rules when it comes to writing a footnote. For MLA, citation footnotes require that you include the name of the author, title of the piece, publisher and publication date on first mention of the source.
Use word processing software to mark each footnote in your writing piece. Select the "insert footnote" option at the end of the sentence or paragraph quoted or paraphrased from your source. A superscript number should appear besides the text and a footnote section with the corresponding superscript number should appear at the bottom of the page. Include a footnote after each resource that was quoted or used on each page. Format your footnote according to your style guide's rules. For example, MLA requires that you insert a tab or five spaces from the left margin before providing the author, publication, publisher and publication date of the piece.
Use a shorter citation for additional mentions of the same source. If using MLA style, insert the last name of the author and the page number(s) in the footnote. If using the Chicago Manual of Style, insert the last name of the author, the title of the piece and page number.