How to Root Samsung Galaxy S2 :

Tools Needed :

1.   Samsung Galaxy S2 Handset

2.   Download Custom ROMs Insecure- Download Here

3.   Download Custom ROMs Original- Download Here

4.   Galaxys2Root.zip  - Download Here

5.   Odin3 v1.83.exe - Download Here

6.   Samsung kies - Download Here

Rooting Steps 

• First, go to Settings->About Phone to find out the model number of your Galaxy S2, you should get something like GT-I9100_XWKF1.

·         Go to this download page and download appropriate insecure and original kernel files. Unzip these zip files. Download Here
·         Download Samsung Kies and install it. 
·      Put your phone into downloading mode by powering your Galaxy S2 off. Then hold down Volume Down, Home button, and the Power button together until you get an option to enter downloading mode. Hit Volume Up to enter downloading mode and connect your Galaxy S2 to your computer via USB cable.

 ·      Run Odin3 v1.83.exe, you should get a YELLOW box displaying COM port. If you don’t, make sure to double check that you have connected your Galaxy S2 to your computer via USB cable and installed Samsung Kies software.

1. ·         Hit the PDA button and find the insecure file you downloaded earlier such as “T-I9100_XWKF1_insecure.tar“

 ·         Hit the Start button and it should start flashing the insecure kernel. This is required so you can get temporary root access to install permanent SuperUser files on your Galaxy G2.

•     When it reboots, make sure USB Debugging is checked on under Settings->Applications->Development then run S2 Root.exe in the galaxys2root folder.

• Hit the “Root Device” button.

Note :

• To Unroot later, run this program again and hit unroot on rooted stock ROM. Make sure to backup your rooted stock ROM!
• Your phone should reboot. Check to see if you have SuperUser app. If you don’t, try running the root program again (Do it again). 
•  If you do have SuperUser app, congratulations! You have ROOTED your Galaxy S2 phone!

After Rooting Your device You must DO IT :

•  You can now reflash the original kernel back. Power off your phone and go back into downloading mode by holding down Volume down, Home button, and Power button then hit the Volume Up button.
•  Open up ODIN program again, hit PDA button, and choose the original kernel file you downloaded earlier such as “GT-I9100_XWKF1_original.tar“.
•   Hit Start.
•   Your phone will reboot
•   If you want to backup your current rooted ROM (I highly advise you to do this) and also install new ROMs, you will want to flash Clockwork Recovery. You can do this by powering off your phone again and go back into downloading mode by holding down Volume down, Home button, and Power button then hit the Volume Up button
• Open up ODIN and hit PDA button, then select GT-I9100_ClockworkMod-Recovery_4.0.1.4.tar file in the galaxys2root folder.
•  Hit Start.
•  Your phone should reboot
•  Any time you want to access your ClockworkMod Recovery, power off your phone then hold down Volume Up, Home button, and the Power button.