Download CyanogenMod image for your phone

Just go here and select your image to download. For the Samung Galaxy S3, I suggest to chose the latest snapshot which is more stable than the nightly builds.

Install Heimdall

Heimdall_ is a cross-platform open-source tool suite used to flash firmware (aka ROMs) onto Samsung mobile devices.

Installing Heimdall (no need the frontend part) :

$ sudo apt-get install libusb-1.0-0-dev

Download ubuntu12.10-heimdall_1.4.0-0_amd64.deb from https://bitbucket.org/benjamin_dobell/heimdall/downloads and install it with dpkg (install with sudo aptitude install dpkg if needed)

$ sudo dpkg -i ubuntu12.10-heimdall_1.4.0-0_amd64.deb

Let's go !

Just boot your Galaxy S3 i9300 in download mode :

Download ClockWorkMod Recovery from http://clockworkmod.com/rommanager (which is a ROM manager and installer)

Link for Galaxy S3 : http://download2.clockworkmod.com/recoveries/recovery-clockwork-6.0.4.6-i9300.img

As CyanogenMod came without Google Apps, if you need them, just download it from http://wiki.cyanogenmod.org/w/Google_Apps

Now launch Heimdall in command line (replace recovery-clockwork-6.0.4.6-i9300.img with the image that you just downloaded) :

Note that sometimes Heimdall command line output some errors like : libusb:error [op_open] open failed, code -1 errno 5 ERROR: Failed to access device. libusb error: -1 Just unplug/plug your phone and retry, it should be okay after several attempts

$ sudo heimdall flash --RECOVERY recovery-clockwork-6.0.4.6-i9300.img
Heimdall v1.4.0

Copyright (c) 2010-2013, Benjamin Dobell, Glass Echidna
http://www.glassechidna.com.au/

This software is provided free of charge. Copying and redistribution is
encouraged.

If you appreciate this software and you would like to support future
development please consider donating:
http://www.glassechidna.com.au/donate/

Initialising connection...
Detecting device...
Claiming interface...
Attempt failed. Detaching driver...
Claiming interface again...
Setting up interface...

Initialising protocol...
Protocol initialisation successful.

Beginning session...

Some devices may take up to 2 minutes to respond.
Please be patient!

Session begun.

Downloading device's PIT file...
PIT file download successful.

Uploading RECOVERY
100%
RECOVERY upload successful

Ending session...
Rebooting device...
Releasing device interface...
Re-attaching kernel driver..

You phone should reboot, Be sure to reboot into recovery immediately after having installed the custom recovery (remove your battery for example). Otherwise the custom recovery will be overwritten and the device will reboot (appearing as though your custom recovery failed to install)

=> Hold Home, Volume Up & Power

Normally you should see the ClockWorkMod logo (see under) with a menu

You can backup your actual system if you want Now Select wipe data/factory reset

Next, you have to install adb (Android Debug Bridge, it's a tool that allow you to open a terminal on your phone from an usb link) in order to put our zip files on your phone

$ sudo add-apt-repository ppa:nilarimogard/webupd8
$ sudo apt-get update
$ sudo apt-get install android-tools-adb android-tools-fastboot

Plug your phone on your computer and launch an adb shell :

$ sudo adb shell

If you see a shell and you can execute command like ls, that's ok, just exit and under ClockWorkMod on your device just go under install zip => install zip from sideload

On your terminal launch :

$ sudo adb sideload cm-11-20140916-SNAPSHOT-M10-i9300.zip

It will install the CyanogenMod ROM, just follow the same process for Google Apps (install zip => install zip from sideload) :

$ sudo adb sideload gapps-kk-20140606-signed.zip