I don't have a smart phone, I've kept my fuel up records for years on note book paper and put the data in when I get to work on my computer. Just remember to write everything down. Fuelly even allows you to add multiple fuel ups one right after the other so I add my wife's car's fuel ups when I remember to collect her credit card receipts out of her center console, sometimes it is a month or more after the car has been fueled before I get the data added.
As bates said, make a note of the odometer reading and how many gallons/litres you put in every time you fuel up, along with the date, in a little notebook or something. Then just punch this data in next time you have access to the internet.
Tip: write the odometer reading on the back of the receipt you get from the petrol station. That way, you have all the information you need on one single slip of paper.