Install Android 4.0 Cyanogenmod 9 Firmware on Galaxy S2 Phone

There is lot of information on Internet about Cyanogenmod, after-market mobile phone firmware installation. However some of this information incomplete and misleading. My first attempt to install Cyanogenmod 9 phone failed. The phone screen played the booting animation in an endless loop. Let me share my installation notes here, this might save you from pulling your hair or worse chucking the phone out of the window.
Disclaimer: Installing third-party firmware will void your phone warranty and if something goes wrong it could make your phone unusable. The installation process will wipe data/apps, Please backup everything.!
Before you get started consider backing up your phone data using Samsung Kies or Android apps such as Titanium Backup. Alternatively you can copy the data manually by connecting the phone to your computer with an USB cable.

Prerequisites: What do you need?

1. Install Android SDK. We'll use the nifty adb program to manage the phone and debug any problems.
2. Install phone flashing program, Heimdall on Linux or Odin utility for Windows OS.
3. Download Clockwork Recovery recovery-clockwork-4.0.1.4-galaxys2.tar. (Please use GT-I9100G_ClockworkMod-Recovery-5.0.2.8.tar file for Samsung Galaxy S2 GT-I1900G phones.)
4. Download latest Cyanogenmod9 from Cynogenmod download page.
5. Download gapps-ics-20120422 from goo.im.This package provides standard Google Android applications such as Google Play (A.K.A Google Market).

Installing Clockworkmod(CWM) Recovery

This is an essential tool for our work. The Clockwork Recovery is modified version of the kernel that provides advanced features such as backup/restore phone ROM's and installing Cynogenmod updates. If you plan to dual boot your phone with different Android versions or Mozilla's Boot2Gecko.
The first step to flash ClockworkMod (CWM) onto your phone using heimdall or 0din program.
Unzip recovery-clockwork-4.0.1.4-galaxys2.tar archive and flash it with heimdall command line flashing utility for Samsung phones.

# Download clockworkmod 
$ wget http://cmw.22aaf3.com/c1/recovery/recovery-clockwork-4.0.1.4-galaxys2.tar

# Unzip it 
$ unzip recovery-clockwork-4.0.1.4-galaxys2.tar


#Connect and reboot the phone into download mode with:
$ adb reboot download

heimdall flash --kernel zImage

Please wait for the phone to reboot. If it doesn't work the first time, try again.
Now you can boot into the recovery mode by pressing Volume Up, Home and Power buttons together until Galaxy S2 logo appears on the phone screen. You can navigation the recovery mode screen by using volume up and volume down keys and power button to select an option.

Backup your Phones original ROM

Restart the phone and enter recovery mode. Select Backup/Restore menu and choose Backup. This provide a safety net, you can always restore the original ROM in case the Cyanogenmod 9 installation fails.

Installing CyanogenMod9 Android 4.0 Ice Cream Sandwich(ICS) on Samsung Galaxy S2

Connect your phone to your computer and transfer the previously downloaded files Cyanogenmod 9 and Gapps files. Restart the phone and enter recovery mode by pressing Volume Up, Home and Power buttons together. Follow the steps below to install the Android ICS on your phone.

1. Select wipe data/factory reset
2. Select wipe cache partition
3. Select Advanced Menu and choose wipe dalvik cache
4. Select Install zip from sdcard and choose update-cm-9-XXX-XXX-XXX-signed.zip file.
5. Select Install zip from sdcard and choose gapps-ics-2012XXXX-signed.zip file. Reboot phone

That's all folks! Don't forget to share your experiences by leaving a comment here.