To add an addendum to a document, open the document in a word processing program, and go to the last page available. In the toolbar menu, use the "insert" tool to create a page break, opening up a new page at the end of the document. In the paragraph formatting section, select the icon that centers the text on the page, moving the cursor to the middle of the page.
Type the word "Addendum" at the top center of the page, press the Enter key, and then write the title of the addendum. Press the Enter key again, and then change the paragraph alignment so that the cursor is moved to the left of the page. Indent between five and seven spaces, and then begin typing the body of the addendum. Once the addendum is finished, save the changes made to the document.
Create a new page for each addendum needed. For multiple addenda, change the title of the page to "Addendum A" and so on, in alphabetical order. For legal documents, ensure that the information in the addendum is accurate and follows legal protocol. If changing or amending a section of the original document, specify what is changing and where it is located on the original document. If adding or responding to a section of the original document, quote the section and indicate its location, as well.