For those of you who want to root your Galaxy S2 GT-i9100 on ICS (Ice Cream Sandwich Android 4.0+), here’s how to do it by flashing a new kernel.
Step 1. First, put your Galaxy S2 into download mode by holding down Volume Down, Center, and Power button together for about 10 seconds.
Step 2. When you see the warning screen, hit Volume Up button to enter download mode.
Step 3. Connect a micro-USB cable from your Galaxy S2 to your computer.
Step 5. Run the ODIN program. You should see a yellow highlighted rectangle box with a number. If you don’t, you need to install Samsung Kies. Download Samsung Kies and install. After installing, un-plug your phone then plug it back in, you should now see it.