by the numbers

Exactly as the rules, directions, or plans dictate. If we all focus and do this by the numbers, then we should have no trouble getting it finished. The military strike was executed perfectly and by the numbers.

In a strict sequence, step-by-step; also, mechanically. For example, The only way to assemble this computer is to do it by the numbers, or Writing a novel is not something one can do by the numbers. This expression has nothing to do with actual numerical figures (like by the dozen) but uses numbers in the sense of a strict order or sequence.

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.