Installing plugins currently isn't easy. We expect you to know how to upload files to a server. If you don't know, a guide on how to upload things can be found in the setup guide which is meant for beginners
You can find a list of plugins that libreevent supports here. Choose one and click it to be directed to the download page.
You can install a new plugin by navigating to the libreevent directory on the server and then by navigating to /backend/plugins/payments and uploading the plugin folder into there. Now, restart libreevent.
You can install a new plugin by navigating to the libreevent directory on the server and then by navigating to /backend/plugins/others and uploading the plugin folder into there. Now, restart libreevent.