I just published three guides to get people started with creating and importing their own custom scenery.
Tilt Brush is the easiest way for beginners to start creating content:
http://steamcommunity.com/sharedfiles/filedetails/?id=850258456
For more advanced users here's a guide for Blender. Most of this should apply to other modeling software as well:
http://steamcommunity.com/sharedfiles/filedetails/?id=849800564
And finally once you have your models:
http://steamcommunity.com/sharedfiles/filedetails/?id=850285056