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