I am currently in the process of planning the deployment of an additional domain controller (DC) in an existing Microsoft Active Directory (AD) domain. I found this Microsoft TechNet documentation that has been helpful as a practical step-by-step checklist.

http://technet.microsoft.com/en-us/library/cc759620(v=ws.10).aspx

In addition to the checklist please take into consideration any variables in your current Information Technology (IT) infrastructure when planning to deploy an additional domain controller to an existing Microsoft Active Directory domain.

Enjoy!