$ python3 -m venv my_env
$ source ./my_env/bin/activate
(You should notice that the console starts displaying the virtual environment's name before your own name and the dollar-sign).
(my_env)$ pip install -r requirements.txt
(my_env)$ python3 -m flask run
* Environment: production
WARNING: This is a development server. Do not use it in a production deployment.
Use a production WSGI server instead.
* Debug mode: off
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)