1. If you use flask without uwsgi or nginx, install them first
https://www.digitalocean.com/community/tutorials/how-to-serve-flask-applications-with-uswgi-and-nginx-on-ubuntu-18-04
2. Get a 'real' certificate through certbot provided by letsencrypt.org
https://blog.miguelgrinberg.com/post/running-your-flask-application-over-https