The first step is to add the certbot repo:
sudo add-apt-repository ppa:certbot/certbot
Next, update the package list:
sudo apt-get update
Next, install Certbot's Nginx package:
sudo apt-get install python-certbot-nginx
Next, request the certificate for your domain(s) you want secured:
sudo certbot --nginx -d example.com -d www.example.com
You'll then be asked if you want to redirect all non HTTPS traffic to HTTPS. I always choose this option which is option 2.
Certbot will get your SSL certificate for the requested domains and setup a cronjob which will run twice daily to ensure your certificates are up to date.