Checkout this wiki and my old script.
#!/bin/sh
# Install from Existing Linux
xARCH=i686
# xARCH=x86_64
URL=http://mirrors.ustc.edu.cn/archlinux/iso/latest
TARBALL=$(wget -q $URL/md5sums.txt -O - | grep $xARCH | awk '{print $2}')
cd /tmp
wget $URL/$TARBALL
tar xf $TARBALL
cp /tmp/root.$xARCH/etc/pacman.d/mirrorlist{,.old}
cat >/tmp/root.$xARCH/etc/pacman.d/mirrorlist <<DELIM
Server = http://mirrors.ustc.edu.cn/archlinux/\$repo/os/\$arch
DELIM
/tmp/root.$xARCH/bin/arch-chroot /tmp/root.$xARCH/
ls -R / &
pacman-key --init
pkill -9 ls
pacman-key --populate archlinux
curl -s -L --resolve 'gist.github.com:443:192.30.252.140' https://gist.github.com/kemadz/2517579/raw | sh -