You need to "add" the domains to your dreamhost account. (Different than registering which you have already done).
Once added each one will be a seperate subdirectory on the file server for example in ftp: /example1.com /example2.com etc. Upload the files to the appropriate sub directory. Then when you browse www.example1.com with your favorite web browser you will see the files you uploaded to /example1.com.
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