This is how I did it. It is quite simple on the 190D, other cars may be different but the concept is the same.

1) I use a laser level like this and set the beam to a line, not a 'dot'

http://www.harborfreight.com/24-inch-digital-laser-level-with-angle-finder-93884.html

2) Locate the center of the rear diff, mark it. On the 190D it is very obvious as it is the lowest point of the diff.
3) Locate a 'cross member' support at the front with a center point. I found one just under the engine with a hole right at the middle. I mark the hole with a pencil and plumb it.
4) Put the laser on the ground and line up the 2 points and that is the center line. I marked both ends on the ground using the laser line.
5) I got the inspiration from this

http://advantagewheelalignment.com/Lazer_Align.html

6) I measured a fixed distance from the center line on either sides and I can now do my alignment. It is quite fun.

Now steering is straight, the alignment is not quite to spec yet.

I need a toe in spec in Degree for 190D. A toe-in spec in inches or mm is ambiguous as it depends on how you measure it, at the rim or at the outside of the tires. The spec I found here is not in degree. One can always use arc-tangent to obtain the degree with the measurements. I find a spec stating the toe-in is 0.33 degree but NO tolerance. Can some one confirm it or have a correct spec?

Thanks

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.