Write out the article and section you want to cite. These are written out in the constitutional documents; every section and article is enumerated (for example, Article II, Section 1 - The President). Write these down on paper so that you will be able to write them out in the footnote.
Write out the amendment you are referring to if you are quoting the Bill of Rights or any of the additional amendments. Amendments are not divided into sections or clauses. When citing an amendment, simply refer to it by number.
You should find an "insert footnote" button on the screen of your word processor. Click on it at the end of your written statement that refers to the referenced constitution. The "footnote" or "footer" option is generally located in the "insert" tab at the top of the page.
Write out the name of the constitution, as well as the article, section and clause. This should read, "The Constitution of the United States," Article X, Section Y, Clause Z (substitute article, section and clause numbers for variables). Amendment citations should read "The Constitution of the United States," Amendment X.
Write the citation out in short hand if you insert additional constitutional footnotes. Short hand for the United States Constitution is "U.S. Const." For the articles section, follow the form "U.S. Const, art. X, s Y, cl Z." For the amendments, follow the form "U.S. Const, am X."