X-Plane itself does not need to be installed before using WED. However, WED will
require that you choose the X-Plane folder before it will allow you to do
anything. Create a working directory with the following structure before
starting WED for the first time.

working directory <-- Pick a name
   |--Custom Scenery
   |--Global Scenery
   |--Resources
         |--default scenery

You can create it with the following commands:

$ mkdir working_directory
$ cd working_directory
$ mkdir -p 'Custom Scenery' 'Global Scenery' 'Resources/default scenery'