I believe this is what I used to get my multi-site instance set up: Multi-Site, Single Core/Manager Setup | MODX Community Forums. I couldn’t find any new tutorials in the docs. 
No extras/mods required.
As long as your DNS records are directing your domains to your host, I don’t think you will have to modify them. Then, based on where the actual sites will live, you will need to point your domains appropriately via your host…it worked way better than I thought it would.
I did run into a snag…But all is working now. 