To install an extension, you must:
- Get an extension from the Magento Marketplace or elsewhere.
- Get the extension’s Composer name and version.
- Update the composer.json file in your Magento project with the name and version of the extension.
- Verify that the extension installed properly.
- Enable and configure the extension.
https://devdocs.magento.com/guides/v2.2/comp-mgr/install-extensions.html