The principle is simple—you go in the opposite direction of all of your natural tendencies when it comes to practice.
First, you resist the temptation to be nice to yourself. You become your own worst critic; you see your work as if through the eyes of others. You recognize your weaknesses, precisely the elements you are not good at. Those are the aspects you give precedence to in your practice. You find a kind of perverse pleasure in moving past the pain this might bring.
Second, you resist the lure of easing up on your focus. You train yourself to concentrate in practice with double the intensity, as if it were the real thing times two. In devising your own routines, you become as creative as possible. You invent exercises that work upon your weaknesses. You give yourself arbitrary deadlines to meet certain standards, constantly pushing yourself past perceived limits. In this way you develop your own standards for excellence, generally higher than those of others.
In the end, your five hours of intense, focused work are the equivalent of ten for most people. Soon enough you will see the results of such practice, and others will marvel at the apparent ease in which you accomplish your deeds.
Mastery by Robert Greene