Put your hand on your forehead and force your forehead against your hand to provide resistance. You can do this in several sets of 6, 8, or 10 repetitions.
You can also do these exercises with a stability ball. For neck flexion, stand facing the wall and hold the stability ball at forehead height. Use your neck muscles to push your forehead into the stability ball. Relax and repeat.
For neck extension, stand facing away from the wall and hold the stability ball behind your head. Push back of head into the ball.