In a prescribed, routine fashion. The term originated during the Civil War, when recruits were taught to load and fire using a series of numerical steps. For example, nine distinct operations were involved in loading a Springfield rifle. The term was revived during World War II for training certain basic operations, such as putting on a gas mask (at the count of one, unfasten the carrier, at two, remove the mask, and so on). After the war the expression was used figuratively in civilian contexts, as in, “In checking out a reserved library book she did it by the numbers—looking it up in the computer, checking the patron’s card, etc.” See also by the book.