The method is simple .. just install the provided zip package from stock recovery. The zip package is created by XDA senior member PhilZ.(thanks for making this).Please check the firmware details..... if your phone have the same firmware below then you can install this zip in your phone..otherwise don't try it.
Firmware Details:
Android Version: 4.1.2 – Build JZO54K
PDA: I9100XWLS8
CSC: I9100FOPLS3
MODEM: I9100XXLS8
Must Note:
- This root package is only for International Galaxy S2 I9100 Android 4.1.2 XWLS8.
- Backup Impotent Data.
- Battery Charge Must be 60% or above.
* Rooting may void warranty or it may brick your phone ..we are not responsible for anything ..do at your risk.