The problem is simple, but there is no explicit “method” when we are in front of a console, over SSH for example.
You have to use the basic functions of Linux, the same way to power off a PC under any of distributions :
… or :
# shutdown -s now
Wait until only one red LED still light on, and unplug the Raspberry’s electrical supply.
On ArchLinux with Systemd, you can run:
# systemctl halt