1. On Run, type dcpromo.
2. On Welcome to the Active Directory Domain Services Installation Wizard, click Next.
3. On Operating System Compatibility, click Next.
4. On Choose a Deployment Configuration, select Existing Forest --> Add a domain controller to an existing domain, click Next.
5.Type the name of existing domain example: contoso.com
6. Click Set, enter the username and password for the existing domain.
7. On Select a Domain, select the domain page, click Next.
8. On Select a Site, select a site page, click Next.
9. On Additional Domain Controller Options page, click Next.
10. A warning message might appear advising you to set DNS, click Yes.11.On Location for Database, Log Files, and SYSVOL page, type or browse to the volume and folder locations for the database file, the directory service log files, and the system volume (SYSVOL) files, and then click Next.
12. On Directory Services Restore Mode Administrator Password page, type and confirm the restore mode password, and then click Next. This password must be used to start AD DS in Directory Service Restore Mode (DSRM) for tasks that must be performed offline.
13. On the Summary page, review your selections. Click Back to change any selections, if necessary.To save the settings that you have selected to an answer file that you can use to automate subsequent AD DS operations, click Export settings. Type the name for your answer file, and then click Save.
When you are sure that your selections are accurate, click Next to install AD DS.
14. On the Completing the Active Directory Domain Services Installation Wizard page, click Finish.
Source from: http://technet.microsoft.com/en-us/library/cc753720(v=ws.10).aspx