I look at my bank statements month by month, and I can tell you to the penny how much we spend.
Related vocabulary: day by day, week by week, year by year
month by month
1. During the course of each consecutive month. We've steadily been growing the userbase month by month.
2. Considering a month in comparison to the one before and after it. I think you need to look at your expenses month by month to see where you can try to save some money.
ˌmonth by ˈmonth
as the months pass; each month: Her pain increased month by month.