In the app, go to "Menu" > "My wallet" and press "Redeem voucher".
Scan the QR code on the voucher provided by the establishment.
If the app doesn't recognise the QR, press "Scanning issues?" and you will be able to the enter the voucher's alphanumeric code manually
Once the voucher has been recognised, press "Redeem voucher".
The balance will be added to your wallet.