I'm looking through the 2015 What's New document on Composer, and it says that Render Mode can be a document property, but only if I create an environment variable called DS_3DVC_OVERRIDE_RENDER_MODE. How do I do this?

Here's the full text:

Render Mode

You can now define the render mode as a document property, or as a default document property, of the viewport.
You set this definition in the Viewport page of the Document Properties or Default Document Properties dialog box.
You must first create an environment variable, DS_3DVC_OVERRIDE_RENDER_MODE, and set its value to 1.
Previously, you could only set the render mode from the Render tab in the ribbon.