This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
You can add Supersized on a single page with a content element and configure it inside the plugin or you can add it via TypoScript for the whole website or a website section. For this way look in chapter Administration => Add Supersized via TypoScript.