> If you are on Linux, OS X or Cygwin, install flask and extensions by entering the following commands, one after another

I think this is a really bad advice. Instead create a file called requirements.txt and add the required livraries, one per line.

Then you can install it with

    flask/bin/pip install - r requirements.txt

Edit: More information in the documentation[0]

[0]: http://www.pip-installer.org/en/latest/requirements.html