Find a quiet space where you will not be disturbed. Close your eyes and think about nothing for a period of 2 minutes. Thinking about thinking about nothing, as well as not being mindful of yourself are actions to be considered a break in focus.
Do this exercise until you can practice it for 5 minutes with less than 4 breaks in focus. After you can do this comfortably, increase your practice time to 10 minutes and practice until you can go the entire 10 minutes with less than 5 breaks in focus.