In iPhones (iOS 11 or newer) and certain Android phones from Xiaomi and Motorola you take a normal photo. If there’s a QR code in the picture, it will be interpreted.
In Android phones you can otherwise use Goggles (often pre-installed) or Google Now! to read QR codes.
For other phones we recommend CliqTags QR Code Reader, as it’s very simple and to the point.
Then read this QR Code to verify that it works:
That’s all there’s to it!