Install Python and python-venv:

sudo apt install python3-venv

Create a virtual environment to run your Flask project:

cd /home/username
python3 -m venv project_name

Install Gunicorn, Flask and other requirements:

source project_name/bin/activate
pip install wheel
pip install gunicorn flask

Create Flask APP file:

app_name.py

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run(host='0.0.0.0')

Install supervisor for observe Gunicorn:

sudo apt install supervisor

Create supervisor conf file:

sudo nano /etc/supervisor/conf.d/project_name.conf

/etc/supervisor/conf.d/project_name.conf

[program:ProjectName]
directory=/home/username/project_name
command=/home/username/project_name/bin/gunicorn app_name:app -b 127.0.0.1:5000 --reload --max-requests 1 --access-logfile=- --error-logfile=-
autostart=true
autorestart=true
stderr_logfile=/var/log/HelloBot.err.log
stdout_logfile=/var/log/HelloBot.out.log

Reload conf file and restart supervisor:

sudo supervisorctl reread
sudo service supervisor restart

Install inotify-tools and Gunicorn will restart automatically when detect code changed:

sudo apt install inotify-tools
sudo service supervisor restart

Caddy Proxy Setting:
/etc/caddy/Caddyfile

example.com {
        root * /var/www/html

        handle /api/* {
        # uri strip_prefix /api  ## uncomment this line to strip prefix
        reverse_proxy 127.0.0.1:5000
        }

        tls yourname@email.com
}

Finally, reload and restart Caddy:

sudo systemctl reload caddy
sudo systemctl restart caddy

Enjoy ~ 🙂