Assuming that you are referring to a blog hosted at wordpress.com, the process is actually pretty simple, and is described for you on the wordpress.com]http://faq.wordpress.com/2006/11/10/domain-mapping/]wordpress.com support site:
Note that this service costs $10.00 per year at WordPress.com, and is called "Domain Mapping".
You make the required changes to your nameservers by editing the whois information for your registered domain from withing the DreamHost control panel. From the "Domains" -> "Registrations" screen, use the "Modify Whois" *is displayed differently depending upon how many domains you have registered) and, on the resulting screen, scroll to the bottom of the page and place the WordPress.com nameservers listed above into the appropriate fields under the "Set DNS info to host your domain elsewhere" section. Save the changes by clicking the "Set these nameservers for yourdomain.com now" link and you are all set.
Per the WordPress FAQ:
--DreamHost Tech Support