To scan a QR code, open the camera app on the iOS or Android device that you use to access the Mettle app and scan the code.
Hold your device so that the QR code is clearly visible on the screen. Your device should recognise the QR code and send you a notification.
From there, just tap on the notification to open Mettle and approve access.
If you don't have a Mettle account, you can get one at mettle.co.uk.