A group of us was carpooling to the beach. A lady my friend knows was bringing her grandson. The grandmother asked us to do the car seat as she doesn't know how. I went to get the car seat from her car and it was in the center position with the center seatbelt and then the passenger outboard seatbelt as well, both through the FF belt path. I thought that the grandparents probably don't know much about car seats, so I explained to her that you only need to use the center seatbelt, and she told me her daughter put it in the car. I know people don't read manuals, but come-on, there's pictures in there and not one picture has two seat belts. I told the grandmother to tell her daughter and if her daughter doesn't believe me, please go see a CPST. After the carpool, I showed the grandmother how to get the car seat in with only ONE seatbelt. Hopefully, grandmother leaves it that way now, as I'm afraid how it will be installed next time.