Righto, the one thing NOT to do (just cos I don’t know what kernel it has, is perform a factory reset, dopesn’t come into it for just unlocking but thought i’d warn you. DO NOT DO ONE not worth the risk – a small number of them have a glitched kernel that rapes its own mind). If installing a custom mod the instructions will tell you to do one, not necessary – there’s a work around.
But for unlocking you don’t need to do that, so don’t worry.
You need SUinstaller for your phone (i9100 or i9100G) – for i9100P use i9100. And the latest Clockwork mod.
Put both those on a microSD card.
Boot to recovery, run .zip from sdcard, find, slect and run CWM, from CWM do the same, and run SU installer. (SU installer won’t run from the stock recovery.
From app store try the free S2 unlockers, didn’t work for me but they’re free so try them first, in the end I payed a small amount for one that worked v quickly.
Feel free to ping me an email, going out now, but back later, I’ve just gone through this and if yours isn’t a G I have the right files too.