It is very easy... copy the following installer to the root directory of one CD... from OpenFirmware type:
boot cd vmlinuz-chrp.initrd

Downloads needed...

Install from CD:
http://cdimage.debian.org/pub/cdimage-testing/sarge_d-i/powerpc/rc2/sarge-powerpc-netinst.iso
http://ftp.debian.org/debian/dists/testing/main/installer-powerpc/rc2/images/powerpc/cdrom/vmlinuz-chrp.initrd

Install from Network:
http://ftp.debian.org/debian/dists/testing/main/installer-powerpc/rc2/images/powerpc/netboot/vmlinuz-chrp.initrd