Easiest way will be to do it as follows:
- Shut down nginx
- Install discourse per official install guide
https://github.com/discourse/discourse/blob/master/docs/INSTALL-cloud.md
- Follow other sites on same server tutorial to create a server block in nginx to forward requests to Discourse and modify the yml to use sockets.
Adding an offline page when rebuilding
The above link should give you a good starting point
- Rebuild discourse with new configuration
- Start nginx