Hello
With SBX you have first to "erase" default" roads, streams, ....
so, you draw polygons and tag with exclude...
First, look at the default cvx file of area with TMFViewer to analyze the existing polys and lines, especially note the end of lines. Better, print it, with different options: one for roads, other for streams, ... For instance, in case of a road,it only needs that your poly intersect the line; if you tag exclude roads, the whole line between end nodes is deleted.
If you plan to create all from start, you can tag "exclude all", but you need to redraw all elements.
If you have FS9 installed and former SBuilder 2: (terrain drawings are often similar in FS9 and FSX); you can import in SB2 project FS9 bgl files (roads, streams, separately), export as bln, and re-import in SBX project; so, you have the default elements drawing, and juste need to make them more accurate with your satellite views. Dafault FSX cvx bgl files unfortunately cannot be disassembled to be edited.
Good luck for your project.
Pierrot - FMEP