you can create web hosting accounts by using your Web Hosting Manager (WHM).
The details of adding and managing your accounts are giving on the page http://sibername.com/support/tutorials.php#whm
These are the tips:
1) Before creating an account, you must create a package. Please use add package link on the WHM to create the packages you will use. you can create different packages with different number of emails, ftp accounts etc For example Sibername has 3 packages for its own use. silver gold platinum. you should create similar packages here.
2) After clicking on add account, enter the domain name, username and password and select the package type from the bottom of the page. you will see that your packages are listed there. the rest of the form will be filled with the numbers that you assigned to the package you selected.
3) make sure the new domain name you added has the same name servers with your main domain name. You can modify the name servers through the domain manager on our web site if the domain is registered with us
4) you can access control panels of your web sites by typing http://www.(yourdomainname)/cpanel
you will have the right control panel by typing the right username and password for particular domain.
Please note that you should close and open a new browser window when you are switching between your domain names.
Because, once you logged in to the address, trying this address will open up a logged in session for your previous domain name. to be on the safe side, close and open a new browser.
Hope this clarifies some details. If you still have questions, please contact us at firstname.lastname@example.org