To set up the service, you'll need either an iPhone or an Android device. Once the app is installed, you've set up your PIN and linked your PayPal account, head to your nearest Shell petrol station (the company says it will roll out on a pump-by-pump basis) and choose how much you think you'll spend filling up your tank. Shell's official guidance states that you can choose between £20 and £150 per transaction, although the app lets you to start as low as £5 and spend up to £100.
In order to reduce confusion, Shell pumps will only dispense fuel once you've told it how much you want to spend and then scanned the QR code. Only then can you lift the nozzle and begin refuelling. When I went to test the app at my local Shell garage, there were plenty of signs and pamphlets explaining how to use Fill Up & Go, but staff explained that it had not yet been enabled at that location.
Shell advises that you stay in your car when scanning the QR code (mobile phones are banned on forecourts), but in reality most people won't want to pull parallel with the code, scan it and then move forward another few feet to align their fuel cap with the relevant pump. However, once you've scanned the code, whether inside or outside your vehicle, you can pocket your phone and fill up as you would normally. Once you reach your maximum limit, the pump will automatically stop serving your fuel and the Shell app should confirm the purchase. It'll also send a receipt too.
Although I wasn't able to successfully test the process from start to finish, setting up the Shell Motorist app and connecting my PayPal account was pretty simple. With its pamphlets, Shell has done a good job of explaining how it works and what people will need. However, whether drivers are comfortable downloading apps and pulling out their smartphone to pay for fuel is another question altogether.