Go to the panel. Then choose "Domains" -> "Manage Domains" and click on the "Add New Domain / Sub-Domain". The top form of that page is for the fully hosted domain.
There you can add "Domain to host". You can freely choose a .dreamhosters.com sub-domain there. Just enter anything like "test123.dreamhosters.com" and when you send the form and that sub-domain is already used by someone else DH will tell you that.
For "FTP user / CGI-runs-as user" you have to choose a user under which the domain will be hosted.
For "Specify your web directory:" you choose the directory in which your pages will be stored on the server. DH will auto suggest to use /home/YOURUSERNAME/test123.dreamhosters.com - just like the domain you added. Instead you could change this and use /home/YOURUSERNAME/therealdomain.com.
You don't need to check the other boxes.
Then submit the form by pressing the "Fully host this domain now!".
Now, if the test123.dreamhosters.com (or whatever you have choosen before the .dreamhosters.com) is free it will be added, if it's already used you have to choose a different one.
After successfully adding that domain you click on "Add New Domain / Sub-Domain" and now add the real domain. The important part here is to use the same user and the same path as for the .dreamhosters.com domain above. This domain doesn't need to be hosted with DH yet.
Now you can build your site using the "test123.dreamhosters.com" domain or whatever you've chosen (it takes some hours for the test123.dreamhosters.com" sub-domain to become alive).
When everything is set you can either change the DNS settings for the domain at the other provider an let them point to:
or you can go to the panel and to "Domains" -> "Reg. Transfer" and transfer the old domain away from you current host to dreamhost completely.
Hope this helps.
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