On your phone or tablet, install the application "Sentinel - your boat, online" which you may download from the App or Play Store (for Android or iOS respectively). Use the app to create an account. Scan the QR code on the front side of the device or provided by the "owner" of the boat.
Upon completion, the wizard will send a "guest request" to the boat owner for confirmation. After the request is accepted you will be able to start using the application.