Make sure that your Galaxy S II is charged to at least 60% in order to avoid running out of battery during the rooting procedure.
You also need enable the USB debugging option that you can find by following this path: Settings > Applications > Development > USB debugging.
You will also have to disable or unintall any anti virus or security software you are running on both your computer and Galaxy S II.
Now things that you must know before rooting your phone: 1.Your warranty will be lost. 2. You will have on boot screen one yellow warning triangle each time you start your phone to announce you to be more careful with the apps that you install from now one.
Get rid of the yellow triangle after rooting: For that yellow triangle you can buy a cheap app from Google play named Triangle Away if it's annoying for you, or you can download it free from HERE from developers official page.
Why you shall root your phone?
-Chainfire 3D (play all games from market even tegra3 games)
ROOTING Samsung galaxy S2
1. Put your Galaxy S2 into download mode by holding down Volume Down, Center, and Power button together for about 10 seconds.
2. When you see the warning screen, hit Volume Up button to enter download mode.
3. Connect a micro-USB cable from your Galaxy S2 to your computer.
4.Download ODIN program and CF Kernel. Then unzip both files.
a)ODIN HERE. b) CF kernel HERE
5.Run the ODIN program. You should see a yellow highlighted rectangle box with a number. If you don’t, you need to install Samsung Kies. Download Samsung Kies and install. After installing, un-plug your phone then plug it back in, you should now see it.
6. Hit “PDA” and choose the “CF-Root-SGS2_XX_XEO_LPQ-v5.3-CWM5.tar” file you unzipped
7. Hit “Start” and the kernel will start flashing to your phone.
8. Your phone will reboot when it’s done.
9. Once you reboot, check that you have the SuperSU app. If you do, you are rooted.
Custom [ROM] ROOT