This article will show you how to add a Windows Server 2012 Domain controller to an existing Active Directory environment with Windows Server 2008 DCs.
With Windows Server 2012 there is no need to prepare the Active Directory Schema before the install.
Dcpromo is no longer available and the installation is done using a new interface.
To start the installation Open Server Manger and Click Add roles and features
Select Active Directory Domain Services
Once the installation is completed it’s time to start the configuration, Open the notification area and Click on Promote this server to a domain controller
Now you will use the new interface to add an existing DC to your Domain or forest.
Click next until you see the install button, from this point everything will be done including the Schema update and a reboot.