Do you still have your manual? The process is written there in detail.
Basically, each assembly has 2 white screws that adjusts the horizontal alignment and the vertical alignment, respectively.
The vertical aim has to be adjusted first. If you are on a level surface aiming the headlights would be easy because all you have to do is align the bubble to 0. The horizontal aim works in the same way, although it uses a "vial" instead of a bubble.