The process is as follows:
Install Windows 2003R2 on the new machine
Assign the new computer an IP address and subnet mask on the existing network
Make sure that the preferred DNS server on new machine points to the existing DNS Server on the Domain (normally the existing domain controller)
Join the new R2 machine to the existing domain as a member server
Take CD 2 from the Win2003R2 disks and insert it in your existing DC and run
NOTE: Adprep is in the \CMPNENTS\R2\ folder on CD2.
Once done you can then remove te CD and go back to the R2 machine.
Now back to the 2003R2 machine – from the command line promote the new machine to a domain controller with the DCPROMO command from the command line Select “Additional Domain Controller in an existing Domain”
Once Active Directory is installed then to make the new machine a global catalog server, go to Administrative Tools, Active Directory Sites and Services, Expand ,Sites, Default first site and Servers. Right click on the new server and select properties and tick the “Global Catalog” checkbox. (Global catalog is essential for logon as it needs to be queried to establish Universal Group Membership)
Install DNS on the new server. Assuming that you were using Active Directory Integrated DNS on the first Domain Controller, DNS will automatically replicate to the new domain controller along with Active Directory.
Don t forget to set the default gateway (router) and DNS Servers. Talking of which all the clients (and the domain controllers themselves) need to have their Preferred DNS server set to the new domain controller.
Check this link for transferring FSMO roles to the new DC.