To install the X-Window System Interface on MINIX, begin by logging into the root account after installation. Ensure your system is internet-connected by testing with a ping. Update the package list using “pkgin update” and install X11 packages with “pkgin in x11”. Wait for the installation to finish before rebooting your system. After booting, log in again with the root account and enter “xdm” in the terminal to load the interface. Finally, log in using your root credentials, with the username “root” and a blank password.