To convert the GUI version of Windows Server 2012 to Server Core:

1. In Server Manager, select Manage > Remove Roles or Features
2. If the Before You Begin page appears, select Next
3. At Server Selection, select the server you want to convert to Server Core and select Next
4. At Server Roles, no changes are needed. Select Next.
5. At Features, expand User Interfaces and Infrastructure, remove the check from Graphical Management Tools and Infrastructure.
6. In the Remove Roles and Features Wizard pop-up box, select Remove Feature
7. Select Next
8. At Confirmation, select Remove
9. Once completed, select Close
10. The computer will need to be restarted to complete the changes. Type shutdown /r /t 0 and press enter to reboot the machine immediately.

Once restarted, you will be in the Server Core interface.