To do something by numbers is to do something exactly according to the prescribed method at every step. There is no 'the' in the expression. From 'paint by numbers'.
To do something by numbers is to do something exactly according to the prescribed method at every step. There is no 'the' in the expression. From 'paint by numbers'.
I think it is more likely to be from the army - where recruits learn all 'complex' actions by being shown them broken down into a series of stages each of which are numbered.