You haven't downloaded the Bancontact App yet? No problem. The Bancontact website provides all the information you need to get started. As soon as the app is installed and configured you can also use it to top up your Mobile Vikings SIM card.
Go to your account and click on the red 'Top up now' button. First select the correct top-up amount, then select 'Bancontact App' as a payment method. You will now be redirected to the page where you will find the QR code to finalize the payment. Scan your code and you’re good to go! ;-)
If you are looking for more information about the exact use of the Bancontact App, consult this page. For problems during the installation of this app, this explanation will get you on the right track.