As root, and sd card inserted (1st bit of thinking, use dmesg to find the device node) (2nd bit of thinking, the 1700 is the size of card, I could have gone higher with the 2Gb used but being safe, an 8Gb card maybe 7700 would be the figure)
Note that the mac address is set in /boot/cmdline.txt and loglevel=2 (smsc95xx.macaddr=08:00:28:00:60:01), and openntpd sets the date for you.
If the card is /dev/mmcblk0
wget http://myplugbox.com/rprootfs.tar.gz
umount /dev/mmcblk0p1
umount /dev/mmcblk0p2
parted /dev/mmcblk0 mklabel msdos -s
parted --align=cyl /dev/mmcblk0 mkpart primary fat32 0 100
parted --align=cyl /dev/mmcblk0 mkpart primary ext3 101 1700
mkdosfs /dev/mmcblk0p1
mke2fs -j /dev/mmcblk0p2
partprobe
cd /media/sd-mmcblk0p2
tar xzfv /root/rprootfs.tar.gz
mv boot/* ../sd-mmcblk0p1/
sync
cd ~
umount /dev/mmcblk0p1
umount /dev/mmcblk0p2
umount /dev/mmcblk0p1
umount /dev/mmcblk0p2
parted /dev/mmcblk0 mklabel msdos -s
parted --align=cyl /dev/mmcblk0 mkpart primary fat32 0 100
parted --align=cyl /dev/mmcblk0 mkpart primary ext3 101 1700
mkdosfs /dev/mmcblk0p1
mke2fs -j /dev/mmcblk0p2
partprobe
cd /media/sd-mmcblk0p2
tar xzfv /root/rprootfs.tar.gz
mv boot/* ../sd-mmcblk0p1/
sync
cd ~
umount /dev/mmcblk0p1
umount /dev/mmcblk0p2
Once installed, you can update with
pacman -Syu
To install say lxde
pacman -S xf86-video-fbdev lxde openbox xorg-xinit dbus
and to start
xinit /usr/bin/lxsession
optional
pacman -S udev-automount