chromium OS

This is easy to read and follow.
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
export PATH="$PATH":thepathto/depot_tools
cd  cr_os

## line below did not work
repo init -u https://chromium.googlesource.com/chromiumos/manifest.git --depth=1 -b stabilize-6436.B

## This works
repo init -u https://chromium.googlesource.com/chromiumos/manifest.git  -g minilayout

repo sync
Now wait until sync is complete.
me@localhost:~/chromimumos$ cros_sdk
[sudo] password for me: 
Attempting download: https://commondatastorage.googleapis.com/chromiumos-sdk/cros-sdk-2014.12.01.203037.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  413M  100  413M    0     0  19.5M      0  0:00:21  0:00:21 --:--:-- 20.9M

INFO    cros_sdk:make_chroot: Unpacking stage3...
INFO    cros_sdk:make_chroot: Set timezone...
INFO    cros_sdk:make_chroot: Adding user/group...

INFO    cros_sdk:make_chroot: Setting up mounts...
INFO    cros_sdk:make_chroot: Running init_setup()...
INFO    cros_sdk:make_chroot: Setting up hosts/resolv...
INFO    cros_sdk:make_chroot: Setting up /etc/make.*...
IN

Scanning Configuration files...
Exiting: Nothing left to do; exiting. :)
INFO    : Elapsed time (update_chroot): 0m35s
Now using icedtea-bin as your generation-2 system JVM
INFO    cros_sdk:make_chroot: Elapsed time (make_chroot.sh): 2m41s

cros_sdk:make_chroot: All set up.  To enter the chroot, run:
$ cros_sdk --enter 

CAUTION: Do *NOT* rm -rf the chroot directory; if there are stale bind
mounts you may end up deleting your source tree too.  To unmount and
delete the chroot cleanly, use:
$ cros_sdk --delete 

(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ 
After this is complete, you land in chroot (as above). You generally cannot run programs on your filesystem from within the chroot. For example, if you are using eclipse as an IDE, or gedit to edit a text file, you will need to run those programs outside the chroot. As a consolation, you can use vim. If you are desperate for emacs, try typing sudo emerge emacs. Of course this command will build emacs from source so allow 5-10mins. Add local user for network less usage!
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ 
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ ./enable_localaccount.sh USERNAME
Enabling local account USERNAME@gmail.com.
Setting CHROMEOS_LOCAL_ACCOUNT in ../../chroot/etc/make.conf.user...
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ 
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ 
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ ./enable_localaccount.sh localuseraccount
Enabling local account localuseraccount@gmail.com.
Setting CHROMEOS_LOCAL_ACCOUNT in ../../chroot/etc/make.conf.user...
Initialise board x86-generic - builds a generic image suitable for computers with a x86-compatible CPU (32 bit) amd64-generic - builds a generic image suitable for computers with a x86_64-compatible CPU (64 bit) arm-generic - builds a generic image suitable for computers with an ARM CPU (32 bit)
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ export BOARD=amd64-generic
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ ./set_shared_user_password.sh
Enter password for shared user account: Password set in /etc/shared_user_passwd.txt
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ ./setup_board --board=${BOARD}
(cr) ((b86854a...)) me@localhost ~/trunk/src/scripts $ ./build_packages --board=${BOARD}
Once the build_packages step is finished, you can build a Chromium OS-base developer image by running the command below from inside the ~/trunk/src/scripts directory:
./build_image --board=${BOARD} --noenable_rootfs_verification dev
Put your image on a USB disk The easiest way to get your image running on your target computer is to put the image on a USB flash disk (sometimes called a USB key), and boot the target computer from the flash disk. The first step is to insert a USB flash disk (4GB or bigger) into your build computer. This disk will be completely erased, so make sure it doesn't have anything important on it. Wait ~10 seconds for the USB disk to register, then type the following command:
cros flash usb://?????? ${BOARD}/latest
cros flash usb:///dev/sdc path/to/image
For more details on using this tool, see the Cros Flash page. When the cros flash command finishes, you can simply unplug your USB key and it's ready to boot from. IMPORTANT NOTE: To emphasize again, cros flash completely replaces the contents of your USB disk. Make sure there is nothing important on your USB disk before you run this command. Building an image to run in a virtual machine Many times it is easier to simply run Chromium OS in a virtual machine like kvm. You can adapt the previously built Chromium OS image so that it is usable by kvm (which uses qemu images) by entering this command from the ~/trunk/src/scripts directory:
./image_to_vm.sh --board=${BOARD}
This command creates the file ~/trunk/src/build/images/${BOARD}/latest/chromiumos_qemu_image.bin. SIDE NOTES: If you want to adapt the image for other virtual machines, see the --format=vmware and --format=virtualbox options. [Note: Only KVM/QEMU VM's are actively supported at the moment, other formats may or may not work on a given build].


Additional information. 

You can get the list of stable builds from here and read this manifest. Other sites of interest: Open source builds of chromium and portable versions. Chromium OS build Another link for chromium OS build Alternatively (this needs to be verified)
cros_sdk -- ./build_packages --board=amd64-generic
cros_sdk -- ./build_image --board=amd64-generic

Now, insert a USB drive (2Gb minimum) & copy the image as follows (note: this will wipe up the drive) :

cros_sdk -- ./image_to_usb.sh --board=amd64-generic