1. Odin 3 Download here

2.Kernels( *.tar, *.md5 or *.smd format) Download here

3.S2 Root Download here

-Settings > Applications > Development > enable USB Debugging

-reboot your device.
-and boot up into download mode by pressing Power+Home+Vol Down. When you reach in download mode,you will see yellow triangle.

-Start Odin3.
-and connect the device to pc via USB cable.(i think Kies should be installed on your pc)

-If your device correctly connect with pc, in the Odin ID:COM should be yellow.

-In the Odin setting,check Auto Reboot and F. Reset Time.

- Press the PDA button and select the Insecure Kernel you downloaded earlier.

-Press the Start button and your device will be reboot.

-When booting up finished, start the S2 Root and
press the root device button.

-The device will reboot again and you done rooting.

If you don't like the yellow triangle at boot up screen, you can flash original kernel with Odin3 again.