https://wiki.ubuntu.com/Core

Deploying Ubuntu Core

Requirements

• Ubuntu Core rootfs
• Linux kernel for target device
• boot-loader for target device

Procedure:

• Format target media: at least one partition should be ext2, ext3, or ext4
• Unpack Ubuntu Core to ext{2,3,4} partition
• Install boot-loader
• Install Linux
• Add a local user and password for the user, then add the user to the adm and sudo groups.

  If the Linux kernel requires modules, add these to /lib/modules/$(uname -r) in the ext{2,3,4} file system
  Boot the target device

  Install any additional required software using apt-get
  

On the SD Card rootfs partition, edit the file /etc/shadow and remove the '*' character in between the semi-colons. You should get something like:

   Edit your /etc/network/interfaces file and add the bottom 2 lines: 

auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp


echo "nameserver 8.8.8.8" > /etc/resolv.conf

/usr/lib/klibc/bin/ipconfig eth0

apt-get update
apt-get install net-tools
apt-get install ppp
apt-get install usbutils