You should see this lever immediately if you look to the lower left of your steering wheel but, if you have some difficulty, don’t hesitate to consult the owner’s manual. The manual will have lots of information about the car’s various parts, including the fuel door release lever.
Once your fuel door is unlocked, you can exit the vehicle and swing the fuel door open. From there, simply unscrew the gas cap in order to access and fill up your mom’s gas tank. Just make sure to fully tighten and close up the fuel tank when you’re finished!