This is not intended to be a tutorial on using GenSurf, but it will include some pointers on getting the most out of the tool. You can download the standalone tool and find tutorials for GenSurf at this site

      This page, in particular, contains tips for using GenSurf, most of which to apply to Q3A terrain creation. 

      When starting the GenSurf plug-in: You will probably want to “Select Ground Surface” as the option for making your terrain surface.