A modus operandi is your usual way of doing things. If your modus operandi is to pretend to have no money to get your friends to buy your ice cream, they already know what you are about to say.
A characteristic way of doing something can be described with the Latin phrase modus operandi, like a friend whose modus operandi is to get rides instead of taking the bus. Your usual method of working is also a modus operandi, like your habit of waiting to study until the night before a test. The police often use the term modus operandi, or M.O., to mean the recognizable methods of a specific criminal.