Setting Up Multisite as Subdomain AFTER Psek has already installed

I am confused/baffled/in need of guidance.

I have a fresh install of multisite from Psek. I asked them to set it up as subdomains instead of subdirectories. They said they couldn’t do that:

“We do not provide wordpress configuration service so regarding this you should better contact your site developers or wordpress community.”

So I come over here, look at the manual, and see that I was supposed to tell Psek to do a Wildcard DNS before the install. Except, on several DIFFERENT occasions, they emailed me and said they couldn’t/wouldn’t help with this.

So now I’m stumped–can anyone guide me through the exact next steps to setting this site up with subdomains? I want to be able to use the domain mapping plugin, and I was told that it would not work with subdirectories–only subdomains (which is why I’m pursuing this so doggedly.)

Any help is much appreciated!