1. in a wide fencer or boxer stance, with knees bent and weight centered evenly (see a good description below on teaching this stance)
2. Keep the neck stiff, and use the waist/back to move the head toward the ball.

    

3. Eyes OPEN!

    

4. Mouth CLOSED!

    

5. Hit ball with the forehead area between hairline and eyebrows.

    

6. If you hit the ball on its bottom half, the ball will go up.

    

7. If you hit the ball on the upper half, the ball will go down.

    

As a general rule, defensive headers should go HIGH, WIDE, and FAR, while offensive headers should be aimed towards ground (as ground balls are harder for keepers to handle). So, defenders usually will aim for the bottom half of the ball, while attackers usually will aim for the top half. Initially, of course, what you really want is the courage to try the technique, so don't get too concerned about where the ball is hit.