Ith has a very high res master image of Chernarus (it's the in-game map with all the topo info, etc.) in his db for the project. On it, he has a bunch of areas highlighted to be mapped and their dimensions in meters. All one has to do is crop the highlighted area you want to work on (if people join the project who don't know how to do this, I'll happily do it for them), set up a CM map with the corresponding dimensions in the terrain editor, and use the cropped image as your overlay. The editor will automatically stretch the overlay image to fit the dimensions on the CM map. For example, while the actual overlay may not be 1km x 1km when it's cropped, it will be proportionally stretched to fit the 1km^2 area on the CM map and you'll be able to map it 1:1. From there it's pretty much just coloring by numbers; adjusting the elevations to match the topo lines on the arma map overlay, putting in a forest where there's one on the arma map, matching up the roads, etc. Give it a go, it's pretty fun!