This tutorial is a quick one. Cause generating an SSL certificate should be quick and painless anyway.
Add the Source
sudo add-apt-repository ppa:certbot/certbot
Install the generator
sudo apt install python-certbot-nginx
Open Up that port
sudo ufw allow 'Nginx Full'
Don't need the lesser one anymore, Full covers HTTP & HTTPS
sudo ufw delete allow 'Nginx HTTP'
Generate the certificate
sudo certbot --nginx -d example.com -d www.example.com
Follow the onscreen instructions the rest of the way, show the EFF some love, and I recommend redirecting all HTTP traffic to HTTPS when certbot asks. Enjoy!