Go to "Petrol Stations" section on the app
A map will pop up marking the petrol stations available for purchasing vouchers near the device location.
Select the petrol station where you want to buy your voucher. You will see that a file will open for the petrol station where you can choose the petrol you want to buy. Select one and a series of boxes with different amounts will appear. Choose the amount you want for your voucher.
This will in turn open a new screen where you can confirm the voucher payment, press "Confirm and pay" and you'll have the voucher available to redeem whenever you like.
Remember that the petrol purchase you have just made is only valid for redemption at the petrol station you have chosen during the purchase process and you can only use it to refuel with the fuel type selected.