You need a DEM (Digital Elevation Model) of the specific area you want.
This can involve piecing together several DEM's to get the correct sized area.....you need 30m resolution DEM's.
For a normal OFP map you need an area 12.8 X 12.8 km.
The DEM's are put together and manipulated in a program such as Microdem.
From Microdem you can translate the DEM into a Terragen file and import it into Wilbur and save as a Lon/Lat text file.
This file can be loaded into WrpEdit or WrpTool (I think).
There is much more to this process, but that is the whole thing simplified in terms of programs involved.
There are other methods, but that is the one I have used most.
EDIT: Beaten by 1 second.......
Planck