[Question] How do you use face masks?

I did buy face masks a while ago, because other rationalists recommended wearing them. The list time I had a flu and went to the doctors office I wore a face mask, the receptionist then told me to take it off while sitting in the waiting room. While I do think I have a better idea then that receptionist, I still don’t know much. It’s easy to have superstitions about how to use face masks correctly. If I want to go for maximum benefit with minimum hassle, what’s worth knowing? When do I throw masks away?