Got this off the web so it must be right, have never tried it myself, but here goes.
You need: long tape measure, chalk or similar, felt pen (not permenant ), a mate (optional), and fingers for doing the adjusting.
Find a flat road surface with a wall, or similar flat vertical surface. Make sure you haven't got a boot full or heavy load in your car.
Back-up 25 feet (in old money) from the wall, and swich on the lights (might be worth keeping it running as the procedure can take a while if your lights are out of alignment)
Measure the distance from the road to the centre of your light.
Go to the wall and draw a horizontal line the same height as you have just measured.
If you think that the lights are not aligned on the horizontal plane, you will need to find the centre line from your car to the wall. It is easiest to do this with someone helping.
Find the horizontal centre of your front windscreen and put an easily visible mark on it with the pen (you could use a small piece of tape instead)
Do the same on your rear screen.
Now, go behind your car and make sure that you are standing in the middle of it. Look through the rear widow, lining up the mark on it with the mark on the front.
Ask your mate to put a mark on the wall where you see the two aligned windscreen marks hitting it. Then take this line directly up/down so it crosses the horizontal line you drew earlier.
Ok, so far so good. Now you need to measure the distance between the centre of your lights (taking the measurement straight - not allowing the tape to follow the car's curves).
When you have the measurement - divide the figure in two. eg if it was 5 feet - then you would have 2.5 feet. Go to the wall, and from the centre point you just marked on it, mark out these distances - e.g 2.5 feet each side. (As you will have gathered, all you have done is to take the measurement of the lights from the front of your car and duplicate them on the wall)
Well, that's it really - you now just have to fiddle about with your lights until you get them to hit the centre of the marks you have just made.
p.s The lights will then be parallel but allowing for the driver should be OK.
If you're flashed either a) you're in luck, b) it's got blue lights and your luck has run out or c) you need to tweak the lights down a bit!
.......... and the best of luck.