1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): by the numbers
Go to by the book.
McGrawhill's American Idioms And Phrasal Verbs