You need to setup the cPanel with primary domain first. The one which would work for public_html. Then click on ADDON DOMAINS. Enter other domain name, set username & password (will be treated as ftp logins for the folder associated with that domain) and then select directory. Its pretty easy if you follow the instructions really. No need to change any name servers. Wait for an hour atleast to see if the domains are redirecting to their respective folders. Also u might need to update the 'A record' of the domains to the IP given to you. Check that in DNS Manager