Adding Domain Controller to forest

Hello,
I work for a company with 5 locations all communicating with each other over ipsec vpn. So far every single piece of hardware is basically standalone. I have installed and configured a domain controller as the first dc and global catalog server. I did this in our hq location. I would like to move onto our second location to install a dc on its network. I was wondering how I go about installing it as a domain controller while also adding it to forest where hq?s domain controller sits. Do I just run dcpromo and install that way? I just want to make sure that all the users and pc?s all tie in across domain controllers. Thanks in advance for the help.