He should understand that when you swing your primary line out away from him or in towards him you are indicating that you want him to turn in the direction you are opening up or closing in. Once he understands all that, you are ready for lungeing. You first establish the feel of a circle by heeding him in a circle, continually opening your primary line to the inside to keep him turning. In the beginning, you can use the walls near the corner of the arena to help you create this feeling. You can also use a round pen.
Round pens make teaching lungeing easier because they help create a circular corridor of pressures.
When the horse develops a strong feeling of a circle, you can start heeding a little farther out from him. As you move out away from the horse, begin gradually dropping back toward his hip in a following motion. As you approach the center of the circle, swing your primary line so that it is behind the horse's withers. This continues giving the horse a feeling that you are following and want him to move forward. You can fuss a little with the lunge whip to move the horse forward but the most important driving aid or pressure is the position of your primary line.