This procedure is available on a number of web sites, as well as in many service manuals. I've tried to take the best of each of the available sources to compose this procedure.
DOT and E-code headlamps require a careful visual aiming procedure to ensure correct alignment. The car should have between a 1/2 tank of fuel and full. Weight in the trunk should be about equal to the normal load (this may be a full trunk, or it may be an empty one, or anything in between), and weight in the driver's seat equivalent to the most frequent driver. All of the tires should be checked when cold to make sure they're at the correct inflation pressure. Bounce each corner of the car firmly (grasp the bumper and push down several times rhythmically) to ensure that the suspension is settled into a normal position.
Park the car on level ground 25 feet away from, and perpendicular to, a vertical wall. The vertical wall should then be marked:
+ o + (c) (v) (c) b------------------------b
+ (c) is the axis, or aim-respective center, of each headlamp. This is usually indicated by a name brand or another mark on the headlamp lens.
If no mark is present, make a careful estimate by looking at the bulb location inside the headlamp. The bulb axis is also the headlamp axis.
o (v) is the centerline of the vehicle. Site through the backglass and windshield of the car to aid in placing this mark accurately.
b-b is a line parallel to and three inches below (c)-(c)
Note that US regulations have recently been modified to permit new cars to be equipped with headlamps very similar to E-code lamps, but with line b-b raised to 2.1 inches (5cm) below c-c. You may wish to experiment with both settings and use the one you feel offers the best performance for your specific driving situation.
+ Draw a vertical line through through the center of each (c) point. Do the same with the o (v) point. You now have an accurate plot on the wall of the locations of the headlamps.
(The (c) and (v) designations are for purposes of clarity in this descriptive article. It is not necessary to draw "(c)" and "(v)" on the wall--just plot the points. Of course, you may use the letters in your aiming procedure if it will help you.)
The upper horizontal edge of the low beam light pattern must be directly on line b-b. This will place the upper horizontal edge of the low beam light pattern 3 inches below the centerline of the headlamp at 25 feet with ECE-spec aiming, and 2.1 inches below the centerline of the headlamp at 25 feet with US-spec aiming.
The "break point" or "kink" in the top edge of the low beam light pattern is where the horizontal upper edge of the low beam light pattern begins to rise to the right or steps upwards (see diagram). Adjust the headlamps so that the kink is within +/- 2 inches of point (c).
After adjusting a high/low beam headlamp in the low beam mode, do not attempt to readjust it in high beam mode. All high/low beam headlamps are meant to be adjusted on the low beam setting only--the high beam adjustment is correct when the low beam adjustment is correct.
HIGH-BEAM-ONLY LAMPS IN 4-LAMP SYSTEMS
These must be adjusted so that the bright, center "hot spot" of the beam is straight ahead of the lamp in both the vertical and horizontal planes. Use the intersection of the horizontal and vertical lines at points (c) as "cross-hair sights" to center the high beam hot spot.
Make sure to cover the adjacent high/low beam lamp when you are aiming its high-beam-only neighbor.