First you need to get OSM data for the zone you want to cover. http://www.geofabrik.de/
If you have a GIS (for instance, QGIS is free and very powerful) you can also directly download a window of data for your desired zone.
Then in ScenProc you follow the steps described in chapter 5.9 of the manual.
Chapter 5.9 is the AFS2 derivative of chapter 5.1 to create autogen from XML OSM data (i.e. with .osm extension).
In case you have shapefile OSM data (.shp) check chapter 5.2 for correct Import syntax. Use QGIS to check your data and identify attributes as described in ScenProc manual
You should use SplitGrid function (cf. chapter 9.2.22) in case you make a large flight zone, typically larger than 1.0x1.0 degree cell, but you should start with a small zone.
Launching the steps (should) create a cultivation.toc file.
Check Rodeo's Airport creation tutorial for the next steps, how to use the cultivation.toc file in AFS2...
Especially you need to create a .tsc file which isn't documented yet.
I picked an existing one and created a phantom airfield to attach my cultivation file. Without the phantom airfield my autogen doesn't show off, I hope the needed structure will be documented soon.
Hope this helps. There are plenty more great functions in ScenProc, just read the manual.
Rodeo started trying out ScenProc, he might consider starting another nice little tutorial of his... who knows.
Cheers
Antoine