If the app is installed on the mobile phone, you have to look for the tank campaign in the “Actions and discounts” and activate it. Then fill up at one of the participating Shell petrol stations and show the voucher in the app at the checkout, where it will be scanned and the savings will then be deducted from the total amount.
Important: Shell informs on its info page that the discount cannot be granted when paying with the Shell Card, with other fuel and service cards or with Shell SmartPay.