A dinner napkin is large – fold it in half, making a rectangle in your lap. Even if it comes in a triangle-shape, fold it into a rectangle in your lap.
A luncheon napkin is half the size of a dinner napkin — unfold it all the way, making a square in your lap.
At the end of the meal when you get up to leave the table, do not refold your napkin (even if you do know how to make one of those fancy cranes or roses). Place it slightly crumpled to the left of your place setting.
Courtesy of Culture and Manners Institute