How to create a wildcard sub-domian?

I am trying to install Wordpress MultiSite and in order to do so it says to create a wildcard sub-domain. How, or can, I do this in Virtualmin? If not, can someone help with setting it up in Apache? I'm not exactly sure where/how to do it. The documentation from wordpress is vague: http://codex.wordpress.org/Create_A_Network

I've found some blogs that say you only need to create a virtual domain in cPanel (I've read here about the difference between Virtualmin's sub-server and cPanel's sub-domain, but I'm getting confused with all the nomenclature). I've noticed mutiple VirtualHosts in Apache that are related to my Top-level Server and the sub-servers related to it, but I'm unsure where to stick the wildcard sub-domain. For reference, I have:

example.com subdomain1.example.com subdomain2.example.com
subdomain3.example.com and so forth

So now I need a wildcard sub-domain.

Below is the step that talks about setting up the wild card domain in the wordpress documentation:

Step 2: Setting Wildcard Subdomains (If this is a Sub-directories Install, skip this step.)

Sub-domain sites work with the use of wildcard subdomains. This is a two-step process:

1. Apache must be configured to accept wildcards. Open up the httpd.conf file or the include file containing the VHOST entry for your web account. Add this line: ServerAlias *.example.com
2. In the DNS records on your server, add a wildcard subdomain that points to the main installation. It should look like: A *.example.com

Any ideas?