Simple steps, Simple to do:
First of all you need to download Odin V1.85 which is the Desktop odin version for rooting your mobile.
Get Odin v1.85 from here:
http://www.multiupload.com/1RTGSE061G
Just extract Odin folder and keep the odin3.ini file in it.
Rooting Samsung Galaxy S2:
Make sure you do it the easier way by using the
CF-ROOT kernel.
Check your mobiles kernel in
Settings>About>Kernel : There should be something like
KJ3 or other code in it for which you have to download the CF root file to root your mobile from
http://forum.xda-developers.com/showthread.php?t=1103399
After downloading the respective file, unzip it and you get a
.tar file. DO NOT UNZIP the .tar file.
Steps to install CF-ROOT kernel (Root your mobile):
1.Download the kernel from here:
http://forum.xda-developers.com/showthread.php?t=1103399
2.Power off phone, and enter in
DOWNLOAD MODE ( Press and Hold the
Vol down + Menu/Centre + Power On keys together, it goes to the download mode and then press the Vol Up key to continue)
3.Connect your mobile to the desktop.
4. Open Odin on desktop and click
PDA and select the downloaded
.tar CF-Root file on PDA.
DO NOT CLICK ANYTHING ELSE or it might just brick your mobile.
5.Only Auto reboot and F.Reset Time which are checked from before will be marked.
I REPEAT, DO NOT CLICK ON PARTITION or anything else.
6.Click
Start.
7.Let the phone reboot
.
After the reboot your mobile will have CWM (ClockworkMod) installed which means your phone is rooted.
Congratulations :-)
Now you are free to install any custom OS you want your beauty/SGS2 to have :)