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Open the Play Store on your device. Tap the Search bar. Type barcode scanner. Tap QR & Barcode Scanner from Gamma Play. Tap Install. Tap Accept. Tap Open.
You can find the Play Store in your list of apps. The icon looks like a shopping bag with the Google Play logo on it. This can be found at the top of the Play Store screen. There are lots of other scanners available you can try as well. Most work very similarly. This button appears after the app has downloaded and installed.