First of all remember that you are the only one responsible if anything goes wrong with your phone.
You will first need to:
1. Root your Galaxy S2 device
2. Install a Custom Recovery such as ClockworkMod recovery
3. Download CyanogenMod 9 stable from here for Samsung Galaxy S2 (carrier version or international version).
How to install CyanogenMod 9 stable on Samsung Galaxy S2?
1. After you download the Samsung Galaxy S2, move it to your device’s SD card.
2. Reboot the Galaxy S2 in recovery mode (Hold Volume Up, Volume Down, & the Power button, continue to hold all three until the screen flashes, then release all buttons).
3. Use the “backup & restore” option to backup all your data. This is necessary in case something goes wrong.
4. Apply the “factory reset” option(only necessary if this is the first CyanogenMod installation)
5. Click on “wipe cache”.
6. In the “advanced” menu find “wipe dalvik cache” and click on it. Go back to the main menu.
7. Click on “install zip from SD card” option and click it. You will be prompted with a message to “choose zip from sd card”. Choose the downloaded zip file containing the Samsung Galaxy S2.
8. Reboot you device.
Wait a little before the reboot is finished. You now have CyanogenMod 9 installed on your Samsung Galaxy S2.
You can also watch this video tutorial that shows you how to install CyanogenMod 9 on Samsung Galaxy S2: