Copy sunrise.php into wp-content/. If there is a sunrise.php there already, you'll just have to merge them as best you can.
Copy domain_mapping.php into wp-content/mu-plugins/.
Edit wp-config.php and uncomment the SUNRISE definition line:
define( 'SUNRISE', 'on' );
As a "site admin", visit Manage->Domain Mapping to create the domain mapping database table and set the server IP address.
Make sure the default Apache virtual host points at your WordPress MU site so it will handle unknown domains correctly. (Need info on cpanel, etc. How do you get them to respond to any domain?)