How to use wildcard SSL cert on a subdomain?

Once you configure your application (mysite.com) to use a wildcard SSL cert. What steps are required to use the wildcard SSL cert on an application say, demosite.mysite.com?

I came over from SiteGround and really didn’t use wildcards, I just generated a cert for the couple of subdomains I had. Am I better off just doing that? Thank you.