Use the Web Platform Installer, available here: http://www.microsoft.com/web/downloads/platform.aspx (if it's not already installed). Use the search bar in the top-right, and enter WFastCGI. The search results will appear and will list available WFastCGI versions for both Python 2.x and Python 3.x. Select the version for Python 3.6 and run the installer.

This installation adds two important components to the required tech stack. The FastCGI module is added to IIS, and the WFastCGI Python code is added to a new Python installation. This new installation will be added at C:Python36, unless there is an existing version in that location (not counting Python versions within an ArcGIS10.X Python installation).

Within this new installation, a file called wfastcgi.py is added in the C:Python36Scripts (or equivalent) folder. This file should be copied into the site folder, next to the app.py file.