I think it's this way:
You buy the domain
Go to the nameservers (Look at other posts you can find them there)
Point it to the nameservers of X10
Go to Addon domains into Cpanel
Fill in the info it requests
Then it should create a folder that is named as your domain name (If your domain name is Starskey.com then a folder starskey.com will be created)
Put in the files there
Wait for the DNS
Done..
I think that's all
