When you're following the format guidelines for a professional APA paper, you need to put a running head and page numbers at the top of each page. The running head is a shortened version of the title in all caps. Getting Microsoft Word to display both at the same time can be tricky.
The easiest way to get the running head and page numbers to appear correctly is to use the APA Paper Template provided on the Writing Support Center. It already has both elements set up and you just need to add in your title. You can copy/paste the assignment into the template after the title page.
If you prefer to do it yourself, see the Formatting the Title Page box on the Writing Support Center.
