When writing a document that must be strictly written in APA style, you may struggle to find the guidelines for writing an addendum. An addendum is an addition to a document noting changes, updates or correcting errors. An addendum is also referred to as an appendix. In APA style you title your addenda as appendices.

Open the document to which you wish to add an addendum in your preferred word processing program.

Browse to the final page of the document. Insert a page break to insert a new page.

Enter your manuscript heading on the first line. Press "Enter."

Type, in your heading font, "Appendix." Center this to the middle of the page. If there is to be more than one appendix, label them with a corresponding letter starting with "A". Press "Enter" twice.

Type the appendix title in the center. Set your word processor to double-spacing.

Indent your first line by five to seven spaces (match the indentation with the rest of your document). Write your appendix.