This is how I usually do my stuff:
-getting the idea (contract request in general), if I did my own projects I researched a lot:
setting,athmosphere,map size, layout, what kind of assets do I need, how can I do them in a smart fashion(modular), etc..
- I do the rought terrain (by imagination, DEM, google source) then and put plain geometry blocks on the terrain for testing my gameplay and see how proportions turn out
- I then replace my block gemotetry with my assets, usually I use the same names so after an export my blocks get replaced by proper geometry
- Then I paint the terrain, place vegetation
- last thing is setting up the lighting/particles and do polishing plus performance optimizing (LE does not feature a lot of stuff on gaining more fps in the editor so this is a small part)
Between the steps its nice to test gameplay and performance with some guys if avaiable.
If you test other stuff like lighting setups you can do a quick terrain, paint it in a neutral way, populate it with some models and change the settings using different skies and light. you can save your .sbx files for later usage if you are digging one setting most. using a photograph or 3d-rendering to match a special look is a nice way too.