In the Toning screen slide the intensity level circle upwards to start your toning session. You can control the intensity by sliding the circle up or down. To pause a session, press the pause button. To stop a session, select the 'Stop' button when the session is paused.
You can also use the buttons on your controller. Press the + button to increase the intensity, the - button to decrease the intensity and the Slendertone Power button will pause (press once) or stop a session (press and hold for three seconds until you hear the tone).