Stand erect, or sit with your back straight and your shoulders relaxed. Release any tension in your neck, throat, shoulders and chest.
Breathe from your diaphragm by using the diaphragm muscle, expanding it to pull air into your lungs. Control your airflow as you sing in the whistle register by slowly pushing air out of your lungs using these muscles.
Relax your lips, jaw and tongue and all the muscles in your face. Open the back of your throat by maintaining proper posture and holding your head up high.
Start on an A-sharp above the staff and sing down a perfect fifth without entering the whistle register.
Sing a B above the staff and feel the change in your throat as you enter the whistle register. Direct your air flow so that you feel only the outer edges of the vocal cords vibrating. Exhaling too much air will prevent you from singing in the whistle register, since too much air will move the entire vocal cord.