Adding a site and subdomains

I installed WP through my cPanel and checked the multisite box. I then added a subdomain in my cPanel (subdomain.domain.com) When I add another site to the network, it comes out as domain.com/subdomain. I need it to be subdomain.domain.com. I have read numerous tutorials on domain mapping and wild cards and still don't get it. Do I have to go in and change the files and DNS like it all says, or is there a plugin that will do this for me? Thanks!