Check with your professor first and ask if they prefer the use of footnotes or endnotes. Note that footnotes appear in the footer of the same page the original work's used, whereas endnotes appear at the end of a section, chapter or at the end of a research paper.
Open up your working document and find the area where you've taken information from an outside source. Insert a parenthesis at the end of your sentence with the original author's name inside. For example, if you used something written by John Doe, then it may look something like this:
Original text of the book as it's written (Doe).
Create a footnote by opening the footer area of your paper in a word processor. Note that according to MLA, you do not need to cite a source if the author's name is mentioned just before including the text in question. Format your footnotes to appear as follows:
Author name, Title of the book, Issue if it's a anthology or magazine, publisher, pages the text appears upon.
Write the author's name and page number if you use their work more than once. For example, if you mention them on page one and page two of your work, the footnote on page two appears as follows:
John Doe, 100-105.
Write endnotes in a research paper or "Cited Sources" within a book by using the same format as mentioned in step three. List each source alphabetically, including the page numbers the original text appears upon within your book or article.
Double check your sources and that you've formatted each one properly before printing out your document. Back up your work on a CD or external hard drive in case you're asked to make edits later on.