The document provides instructions for adding a custom background image to Google Docs documents. It is a 5-step process: 1) Upload the background image to Google Drive, 2) Create a publicly accessible URL for the image, 3) Install the Stylish browser extension, 4) Use Stylish to edit the background CSS for Google Docs pages, 5) Replace the default URL with the image URL. Following these steps allows users to customize the background of shared Google Docs.