Once you have created an account on your server, you (or your customer) can then login to cPanel and manage it. The easiest way to access cPanel is:
http://www.example.com/cpanel
Replacing example.com with the account you just created.
If the nameservers change is yet to propagate then you can also login at:
http://yourserverIP/cpanel
Replacing yourserverIP with the IP address of your server.