CyanogenMod is no doubt one of the best custom ROM for any Android device. I was waiting for a stable release from CyanogenMod for Galaxy S2. Latest stable version of CyanogenMod 7.1 is having Android 2.3.7 Gingerbread with lots of tweaks and improvements. Here is the complete guide on how to download & Install CyanogenMod 7.1 on your Galaxy S2.
** Please note that CyanogenMod 7.1 is a Custom ROM and it will void the warranty of your handset. Install at your own Risk **
- Rooted Samsung Galaxy SII device. Check How to root your i9100 device rooting guide here
- ClockworkMod Recovery which will be there on your rooted device
- Download the latest Stable build of CyanogenMod 7.1 for the Samsung Galaxy S2 (SII) from CyanogenMod website.
- Place the update-cm-7.1.0-GalaxyS2-signed.zip file on the root of the SD card.
- Boot into the ClockworkMod Recovery.
- Once the device boots into the ClockworkMod Recovery, use the side volume buttons to move around, and either the power button or the trackball to select.
- Select the option to Wipe data/factory reset.
- Then select the option to Wipe cache partition.
- Select Install zip from sdcard.
- Select Choose zip from sdcard.
- Select the update-cm-7.1.0-GalaxyS2-signed.zip file.
- Once the installation has finished, select Go Back to get back to the main menu, and select the Reboot system now option.
The Samsung Galaxy S II should now boot into CyanogenMod.
** Installing a Custom firmware is a very risky procedure.. Do it at your own RISK.. **
We have given this procedure on request from readers. However if you are not a very expert user of phone, We will not suggest installing a CustomROM.
Source - CyanogenMod Wiki page