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 Google Now! to read QR codes.
Snapchat supports both its own Snapcodes and QR codes.
We also recommend CliqTags QR Code Reader.
Then read this QR Code to verify that it works:
That’s all there’s to it!