Go to your Domain Registrar
Select your new domain and click on Domain Name Settings
Click on the Manage Nameservers/ Manage DNS
You should see several Text fields like this:
TO POINT TO BLUEHOST ON REGISTRAR SITE
Nameserver 1 ( Add 'ns1.bluehost.com' here)
Nameserver 2 (Add 'ns2.bluehost.com' here)
Nameserver 3
Nameserver 4
Nameserver 5
TO POINT TO DREAMHOST ON REGISTRAR SITE
Nameserver 1 (Add 'ns1.dreamhost.com' here)
Nameserver 2 (Add 'ns2.dreamhost.com' here)
Nameserver 3 (Add 'ns3.dreamhost.com' here)
Nameserver 4
Nameserver 5
Fill in the fields. Bluehost requires you to fill in two fields, while dream host requires you to fill in three fields.
Click Save now.
PLEASE NOTE: Its important to update your nameservers on your registrar immediately upon purchase! Delays can lead to nameserver complications.