In iPhones (iOS 11 or newer) and newer Android phones from at least Samsung, Xiaomi and Motorola you scan QR codes via the camera application. If there’s a QR code in the preview, it will be interpreted, and in the case of a URL/link you can open that site.
Snapchat supports both its own Snapcodes and QR codes.
We also recommend CliqTags QR Code Reader, and there are many other QR (and barcode) apps you can install with different features.
Then read this QR Code to verify that it works:
That’s all there’s to it!