There's a couple of guides to do it various ways. I'm not on a pc or I'd share my links I have bookmarked, but perhaps a rough guide on doing it.
Use a one-click install for WordPress. Set up your WordPress site.
Install Nginx on your server, configure to replace Apache.(optional!)
Install NodeJS and all you need to run NodeBB using below link.
Long, but best way:
Follow above guide or another to install NodeBB. Then for WordPress, you'll need to add PHP and other necessities via the console to your lemp stack, drop WordPress in a directory, point Nginx to it, install your blog.
The former method is far simpler if you don't want the hassle of trying to work through issues with installing WordPress on a server not configured perfectly for it (which there's always some), the latter gives more control and understanding of the setup. It also means you'll be able to identify problems quicker if you know where things are.
If doing the quick way, replacing Apache with Nginx is optional but recommended. Either way you'll have to configure either to point your subdomain and domain to the correct places. Not difficult as there's loads of topics covering them on here.
Sorry this isn't in great detail, I'll get on tomorrow with links for you.