Begin by sitting in a chair with good, neutral posture.  Your shoulders should be over your hips and your head should be facing straight ahead.

While keeping your head level, slowly retract your chin and glide you head straight back over your shoulders.  You may use your hand to either guide your head or press on your chin to get more extension.  After you have gone as far as possible or began to experience any increase in pain, return to the starting position.  Repeat this attempting to get farther on each repitition.