I have decided to make a guide that will hopefully help beginners to root your phone and be on your way to flashing a custom ROM.
There are many ways in which to root your phone, but I think the easiest way is through Chainfire’s CF-Root, which is a kernel that basically roots your phone, adds Busybox and ClockworkMod Recovery (this is necessary for installing/flashing custom ROM’s).
First off, you need Odin, which is a flashing tool specifically designed for the SGS2: Just type in "Odin 1.85" without quotations into Google and the first result should display "Odin - xdadevelopers - XDA Developers." Click on the link and Odin will download.
Now before going ahead with Odin, download Kies: http://www.samsung.com/us/kies/ ,which is like iTunes for Samsung Devices, the drivers are needed for Odin, but when the drivers have been installed, close Kies, right click on your toolbar at the bottom of your desktop, I'll assume your running Windows 7, and go into Task Manager. Now find anything with Kies running in the Task Manager, right click on them and kill them.
Kies gets jealous of Odin for being better, so it messes up Odin during the flashing process. So make sure you have the drivers installed from Kies and then killed off the Kies Processes.
Finally, lets get the flashing package required to root your phone. Here is the link to Chainfire's thread: http://forum.xda-developers.com/showthread.php?p=20075898
Find the kernel that matches your Kernel Version e.g. Under Settings > About phone > Kernel Version: 18.104.22.168-I9100XXKG1-CL349526
What matters most (KG1) in this case, is in bold. Try to find the matching file under downloads CF-Root-SGS2_XX_OXA_KG1-v4.1-CWM4.zip See the resemblance?
Also make sure you unzip the file to get your .tar file, Odin won't work with .zip files.
Now go ahead and put your phone into download mode. To do this, turn off your phone normally, wait till it is fully off, then hold down the power, home and down volume button until a warning screen is displayed. Then press the up volume button. If the screen says "In download mode. Do not turn off target" then you have got it!
Now fire up Odin with Administrator Access (right click on the Odin icon and select Run as Administrator). When you have opened it, installed the Kies Air drivers and killed Kies processes off in Task Manager, you’re ready to proceed. Now connect your phone to your computer via your micro USB cable. The ID:COM box should go yellow and say ADDED!! in the box at the left hand side. Now make sure that the only options ticked in Option are Auto Reboot and F. Reset Time AND NOT RE-PARTITION. Now click on the PDA button on the right, and this will open your Explorer window up. Find your CF-Root file, and click OK. Then just click Start. In the top left hand corner, one of the boxes will flash blue, and then Green. The bottom left hand box will say PASSED!! Your phone will automatically reboot.
When it has, go to your app drawer, and check you have something called either "SuperUser" or "SuperSU." If you do, congratulations, you’re rooted. Another way to check it has been successful is to shut down your phone, and hold the home, power and volume up button. If you get into ClockworkMod Recovery, you’re great!
Now that you have root and ClockworkMod Recovery, the first thing you should do is boot into ClockworkMod Recovery by shutting down your phone, and using the home + power + volume up button sequence. When it loads, use the volume up and down buttons to navigate, and the power or home button to select depending on your CF-Root version.
Go to Backup and Restore, and then go to Backup to external sdcard. This will make a clone of your entire ROM, which is crucial if something with your custom ROM goes wrong, at least you can return to a point when you know the phone works.
After this, go to the Android Development Thread, and download a custom ROM you look the like of. Most will be in the .zip format, so Odin won’t be necessary. Just put the file on your sdcard, boot into ClockworkMod Recovery, make the necessary wipes outlined on the ROM’s thread, then Install zip from sdcard, then choose zip from external sdcard, find the ROM file, and then yes. MAKE SURE YOU FOLLOW THE INSTALL GUIDELINES ON THE ROM’s THREAD.
That’s you! When the phone is rebooting for the first time, the boot will take considerably longer as your phone is rebuilding the dalvik cache that you most likely wiped during your ROM install. Nothing to get worried about. If you wish to return to your stock ROM, boot into ClockworkMod Recovery, go to Backup and Restore, and restore from an external sdcard. Voila!
If I have left anything out, or you have a question, please post below!
A big thanks to Chainfire who made the root process oh so easy for the SGS2!
DISCLAIMER: I am not responsible for bricked phones or your phone causing a zombie apocalypse, however this process is very safe if followed correctly.
You think CF Root is easy for Noobs?
Easiest way is Flashing from Stock Recovery
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Sent from S II || Running Official ICS 4.0.4 with Siyah 4.1