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.
Install the extension using your preferred method, TypoScript include is handled automatically.
Head over to your CLI and run the following command
composer require yoast-seo-for-typo3/yoast_seo
After installing it’s necessary to flush the system caches and possibly the “Compare Database” section of the install tool.