Release any tension from the muscles in your face and neck.
Open your mouth in a relaxed manner by dropping your jaw and slightly tilting your chin inward toward your chest.
Hum an "oooo" sound at a pitch that seems natural. Don't even plan the pitch. Just produce an extended tone at a comfortable volume level.
Inhale and produce the same pitch. This time, as you produce the pitch, begin to slide upward with your voice so that your sound becomes high. Don't strain your voice, but imagine that you are creating a child's "oooo" sound, as if the child has just seen something amazing.
Inhale again if necessary and slide your voice back down. This time try to go lower than your original pitch.
Record yourself changing your vocal pitches. Review the recording to see if you can hear the changes in pitch.
Play any key near the center of a piano or keyboard. Hum that note. Play the next five to eight notes to the right of the original note and hum along. This will cause you to raise your pitch. Go back to the original note and play the next five to eight keys to the left of the original note in order to lower your pitch.