You'll also insecure kernel, you may need from here. Although it is recommended that you have kernel code, so close to get one phone already (check this, go to Settings> Applications> About phone, and you'll see it under Kernel version), the not required if you can reload at a secure kernel when the rooting procedure is all done and dusted.This is important. Only if this feature is enabled on your phone, you will not be able to continue. Go to Settings> Applications> Development and make sure the 'Enable USB Debugging' check box.
Once this is done, give yourself a pat on the back and go to Step 3. Turn off your phone. Keep pressing the Volume Down button, Home button and Power. The phone will reboot and you should see a screen warning of dangers to install custom software OS. If you are happy to proceed at your own risk, press the PTT key to the phone in Download mode and you will be presented with Andy the Android in all its splendor luminous green. Use USB cable to connect your Galaxy S2 to your computer. Enter the freshly Odin3 application downloaded on your computer. You should see that the box in the upper left yellow Odin interface, which means that your phone has successfully identified. You must now check that only the "Auto Reboot" and "F. Reset Time" check boxes in the Odin site.
Click the PDA button in Odin3 and select the kernel you got earlier. Take a deep breath and click on 'Start'. When the process is complete, your phone turns itself off and should give Odin3 green "PASS" message on the screen of your computer.Your phone may reboot itself, but if it shows the battery charging icon, you must enable it manually. When the boot screen appears Galaxy S2 standard, you should have a yellow triangle with an exclamation point at the bottom of the screen, indicating that you are using insecure kernel (you already know). Keep your phone connected and wait for it to reboot itself properly. Once it is, start S2 Root. Which, if you are awake during this guide, you will have already done - you'll be a small warning message asking if you are uncertain USB debugging kernel flashed and get activated. Click OK.