Here is the first part detailing how I work with World Machine. It gives and overview and goes further into Extents.
You can download the file I'm using here:
It is a corrected version of the previous file. I've removed some of the clutter, but kept all the vitals. This file only has elevation color and no textures. But all the terrain generating functions are there as well as Rivers and Lakes.
Part 2 will cover Base Terrains.