In your Terminal or Command Prompt go to the folder that contains app.py. Run the following command:
python app.py
It will start the server at http://127.0.0.1:5000. The link will open automatically on your web browser. Do not close the Terminal or Command Prompt while you are working on the web application.
The application will create Accounts folder inside the root directory of the project. This folder contains the entire data to be saved.