You need to have:
1) Android ICS 4.0.3 installed
1 ) Put the SuperUser zip to your sd card
2 ) Turn off your device
3 ) go to Bootloader (press&hold volume up button then press&hold power button)
4 ) when in Fastboot mode navigate to [Recovery] using volume up/down buttons
5 ) Press the power key to select [Recovery]
6 ) If you see a warning logo the you have not flashed the ClockworkMod Recovery image properly, so go back and do that, then start again from 3)
7) From the menu, select [Apply update from /sdcard], and choose the SuperUser zip from the list(with Power button).
8) When update is finished go back and select [reboot system now]
9) if you want to disable automatic reset of the original recovery you may rename ‘/etc/installrecovery.sh’ to smth else like ‘installrecovery.sh.old’. I would not recommend deleting it as this is the easiest way if you want to unroot the phone and go back to the stock rom.
10) Enjoy your root but be careful 8)