To install Flask, you need to have Python 3 installed first. Once Python is installed, use Command Prompt to navigate to your Python/Scripts folder, then type:
On MAC (as of mid-2018) - running two simultaneous instances of IDLE takes some effort. For MAC only, I am using IDLE to serve web applications. To simulate a client on Mac, and to have a console window to print to for debugging purposes with your client (as your IDLE console window will be busy serving your web application), I would suggest installing Thonny. This way you can debug both server (via IDLE) and client (via Thonny) simulataneously. Run your web server via IDLE, and your client via Thonny.