Download the powerpc ISO
https://archlinuxpower.org/iso/
Although there are ways to install via USB, we assume you burnt it to a CD/DVD and boot of your optical drive.
Create a bootstrap partition
It should be at least 4MiB, read not the default 800KiB size!! Depending on which Grub modules you'd like to generate into the Grub boot image, it may need to be bigger. 32MiB is plenty and you'll likely won't ever need more. (That's similar to what they said about the Apollo Guidance Computer's amount of "RAM" too though).
Install base using pacstrap
Modify the target folder to your needs, we assume /mnt here and add kernels, grub and hfsutils for good measure.
If on a 64bit machine such as a G5 or POWER6, consider installing linux-ppc64 for the kernel.
$ pacstrap /mnt/ base linux{,-g4} grub hfsutils
Format bootstrap partition
We assume it's the first partition (after the Apple partition map) thus /dev/sda2:
$ hformat /dev/sda2
Mount bootstrap partition
Still we assume it's /dev/sda2 an you need to mount it to /boot/grub on the target rootfs, add it to fstab along with the rest of your partitions and install Grub.
$ mkdir /mnt/boot/grub
$ mount /dev/sda2 /mnt/boot/grub
# bootstrap partition should always get mounted to /boot/grub so grub hooks may work properly
$ genfstab -U /mnt /mnt/etc/fstab
$ arch-chroot /mnt
$$ grub-mkconfig -o /boot/grub/grub.cfg
# this one will fail if bootstrap is too small (depends on Grub mods included with core.elf)
$$ grub-install
Troubleshooting
Additional steps needed to fix and issue where OpenFirmware may not recognize grub, blessing binaries on our bootstrap partition.
$ umount /dev/sda2
$ hmount /dev/sda2
$ hattrib -t tbxi :grub
$ hattrib -b :
$ humount
After you're done with the above, your system should detect the installation and boot Grub. So just configure as normal using the regular Archlinux Installation Guide skipping bootloader specifics.