This is not as difficult or time-consuming as it might appear.
First, you need a topographic map, at least 1:50,000 scale. Of course 1:24,000 or less is even better. You can find plenty on the internet. You can also purchase them in paper or digital versions. I've got a massive amount of topos available through my own collecting and because of my map making for HPS Simulations, so finding something interesting wasn't difficult for me.
Starting with a fairly extensive urban area on my Bluefields, Nicaragua map has tempered my enthusiasm for such projects, so I've decided to select a more rural area dotted with small towns easy to replicate in Visitor, with mixed open terrain and scattered forest to keep up the frame rate while playing. The interest in the Cold War, coupled with the fact that I served in the US Army in 1980's in Germany makes it "no-brainer" for me to pick out a map from central Germany.
A scan of the map area I choose to depict:
I scale the scanned image so each pixel equals one meter. This is a 5120 x 5120 map.
Zoomed in you can see the contour lines - 20 meter base with 10 meter intermediate lines.
Determine the high an low points on the map, and assign greyscale values to the contour lines to correspond. Since the elevations range from 410 to 490 meters, I use an RGP value of the elevation minus 400 to keep it simple (ie elevation 450 is assigned color value R:50 G:50 B:50). The max and min height in the PBL file define the actual in game elevations of course.
I create a separate layer for each contour which makes finding and correcting mistakes easy.
The areas enclosed in contour lines are filled with the same color.