Part of this depends on where your domain is registered.
In any case, you'll have to go to the Panel and edit DNS for your domain:
Add ns1.yourdomain.com as an NS entry that points to ns1.dreamhost.com
Do the same for ns2 and ns3. It may ask you for the IP addresses as well, so you'll have to have them handy for ns1.dreamhost.com, etc.
If your domain is registered at DreamHost, then it's pretty easy to just update your registration info to use ns1.yourdomain.com, etc., without a fight.
If you're registered elsewhere, you'll have to make ns1.yourdomain.com authoritative for your domain before you try to set your DNS servers to your own. At GoDaddy, when you edit your Domain, in the lower left, there's a Host Summary box where you'll need to add your personal name servers before you can change your domain's DNS server.