Choose a topic which occupies your mind frequently. For example, if you usually play videogames for an hour a day, choose video games. Try to avoid topics that you must confront. An example might be avoiding thinking about school when you are a student. Avoiding thinking about work when its not work time.
You should go about your day trying to avoid thinking of this topic. Make note every time you think of this topic,as this is to be considered a break in focus.
Note: This exercise can be very difficult for some people. Try not to go on to the next exercise until you have made a few earnest attempts to get this right.