How do I insert a page border

I need to add a page border to a word document

Format - Page - Borders

Or: Create a box, expand it to fill the page. Set colour to None, this leaves the edge showing, which you can change colour or thickness with tools top left. Then send it to the back. You do have the option to add colour or pattern to the box as a background.
Or you can do it the other way.

Go to insert> format> border