It sounds like you have the procedure correct.
During the sign-up process you will see the option 'I already own a domain, I just want to host it with you', select this option and enter the domain name in the place provided.
Once your account has been approved you will receive an email telling you how to log-in using FTP. When you do log-in you will see a directory with the same name as your domain, this is where your site files go.
When everything is in place, you just configure your domains DNS to the DreamHost DNS servers (or ask your registrar to do this if they don't provide a configuration page). After a day or so, the new DNS information should have propagated around the Internet and your new site will be active.
If you wish to test your site at DreamHost before transferring the DNS, then follow the instructions in the wiki article that Norm linked to above.
I should also point out that all DreamHost shared hosting plans come with a 'free' domain registration for the life of the hosting. Normally you would register this domain during the sign-up process, but you can do it at a later time if you wish.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )