Updated: 2019-02-27
Install certbot.
sudo add-apt-repository ppa:certbot/certbot
Install Certbot's Nginx package.
sudo apt install python-certbot-nginx
Allow HTTPS.
sudo ufw allow 'Nginx Full'
Disable HTTP.
sudo ufw delete allow 'Nginx HTTP'
Obtain an SSL Certificate.
sudo certbot --nginx -d example.com -d www.example.com
Verifying Certbot Auto-Renewal.
sudo certbot renew --dry-run
For a more detailed explanation: Check out these tutorials.
Secure Nginx with Let's Encrypt