This is a two-step process:
Step 1 - Add Active Directory Domain Services Role (steps)
This adds the role to the server to support Active Directory Domain Services. The steps also require the installation of .NET 3.5. When you run the wizard to add the role service, Server Manager prompts to perform the installation for .NET 3.5 if necessary. For a Microsoft article about Active Directory Domain Services Role, see Active Directory Domain Services Role.
Step 2 - Run dcpromo.exe (steps)
This command executable can install or remove Active Directory Domain Services. In the context of this documentation's example, Dcprome.exe is used to install AD DS. For Microsoft's reference for Dcpromo, see Dcpromo.