The basic procedure is to add A records for your domain, pointing to the IP addresses of the DreamHost DNS servers.
ns1 A 22.214.171.124
ns2 A 126.96.36.199
ns3 A 188.8.131.52
Note: The IP addresses of the DreamHost DNS servers may change in the future, so you will need to keep on top of that.
Of-course, after the A records are added and active, you will need to change the DNS servers for your domain from ns?.dreamhost.com to ns?.yourdomain.com. If I recall correctly, there is an extra confirmation step when doing this, where you have to re-enter the relevant IP addresses.
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