Note: It is highly recommended use a test/staging environment to test 3rd party plugins with your customizations and see how your website is affected, before installing the plugin on your live website.
1. From WordPress left menu, go to Plugins > Add new.
2. Search for a plugin by typing a plugin name into Search Plugins input field.
3. On the plugin you wish to install, click Install Now.
4. After the plugin is done installing, click on Activate.