You can puzzle the steps together by following my guide about setting up an offline page:
The guide assumes you start out with Discourse not running behind nginx and guides you through setting up nginx including Let’s Encrypt and setting up an offline page. You should be able to simply skip what you’ve done already, and stop as soon as the offline page work starts 
(The core idea is to set up Certbot on the host, configure nginx to redirect most HTTP traffic to HTTPS but answer acme-challenge queries from the webroot, and teach nginx to use the resulting cert.)