This article provides instructions for adding add-ins to SharePoint Online or Project Online sites. The steps include logging into the site, clicking the gear icon to access the settings menu, selecting "Add an app" which will redirect to the Your Apps page, clicking "SharePoint store" to access available add-ins, searching for and selecting the desired add-in, clicking "trust it" to complete the installation, and verifying the add-in is now available on the Site contents page.