[Galaxy S II Q&A, Help & Troubleshooting] KayvinM:
Button Combos:

Download Mode: Volume Down+Home+Power

Recovery Mode: Volume Up+Home+Power

Once in recovery use volume keys to navigate, home button to select and power button to go back

Official Firmware Version

PDA: I9100MUGKG2

Phone: I9100MUGKG2

CSC: I9100MBMCKG2

Product Code: GT-I9100LKMBMC

Unlock

Galaxy_S Unlock App by Helroz found here https://market.android.com/details?id=com.helroz.galaxysunlock&feature=search_result.

Rooting

CF-Root Thread: http://xdaforums.com/showthread.php?t=1103399

We do not have a CF-Root version of our kernel so just flash any kernel using the instructions provided in the link and flash the stock kernel back (found below). Root will remain as well as the CWM app but you will loose clockwork mod recovery.

Alternative method:

Zergrush exploit: http://xdaforums.com/showthread.php?t=803682&highlight=zegrush

Yellow Triangle Removal

Tutorial can be found here: http://xdaforums.com/showthread.php?t=1193091

Alternatively you can use a Jig as it also resets the custom Rom counter

I9100 Secret Codes

http://xdaforums.com/showthread.php?t=1193285

All links below are for Gingerbread 2.3.3 MUGK2

Stock Bell/Virgin Modem

Directions: Use Odin and place .tar file in "phone" section

Download

Mirror: Download

Stock Firmware

Download

Mirror: Download

Stock Kernel

Directions: Place .tar file in PDA section using ODIN

Download

Mirror: Download

Bell CSC

Directions: Flash with Odin in CSC section

Download

Mirror: Download

Additional Modems:

Some users are experiencing better Gps/Reception with the KH3/KI3 modem

Directions: Unzip file and flash modem.bin with Odin under "phone" section

I9100XXKH3-Download

I9100XXKG6-Download

I9100XXKI1-Download

I9100XXKI3-Download

I9100XXKI4-Download

*CWM Compatible Modems can be found here

Flashing Instructions​

How to install the latest firmware on your phone. This will also get rid of Bell's bloatware:

1) Make sure you have working drivers installed on your computer. If you do not, the easiest way is to download Kies and it will install it for you.

2) Install Odin 1.85 on your computer (attached at the bottom of this post)

3) Pick any firmware version you would like to flash from Intratech's thread linked here. The firmware is arranged from oldest to newest so the last ones are obviously going to be the newest.

4) Start Odin in "administrator" mode and make sure Kies is not running in the background. Use task manager to check

5) Place your downloaded firmware .tar or .md5 file into the PDA section in Odin. Make sure re-partition is not selected. Auto reboot and reset time should be selected by default.

6) Remove any external sd cards and the sim card from your phone (precaution) then reboot into download or hold volume down + home + power key. If a warning pops up press the volume up key to continue.

7) Connect your phone to the computer with the USB cable. You should see the I.D:Com port turn yellow.

8) Once you are certain everything is where it should be, select START. Once the process has completed your phone will re-boot. Exit out of Odin. That's it!

Optional (Rooting)

9) If you wish to root your will need to download a cf-root kernel that matches the firmware you just flashed from here. For example if you flashed XXKI4 then you will need to download the cf-root kernel that ends with XXKI4

10) Restart Odin and place the kernel .tar file in the PDA section, connect your phone and press START.

Alternative TWlaunchers​

Since upgrading to 2.3.4/2.3.5 firmwares, some (including myself) have noticed some lag/stuttering when flipping through homescreens. I find that this gets worse when using the program monitor widget. I put together some alternatives that I pulled from the US variants of the Galaxy S2.

I have tested them and they have pretty much eliminated the issue. I personally find them much smoother. These are stock Twlaunchers with no mods. The files can be found at the bottom of this post. Requires Root.

Directions: These files a deodexed but will work with Odexed Firmwares (I tested on 2.3.5) Make sure to download another launcher and use it as your primary. While using an alternative launcher, back up the twlauncher.apk and .odex files under system app then the libglcanvas.so under system/lib. Then delete twlauncher.odex. Boot into CWM recovery and flash the new twlauncher. If you do not have CWM recovery you can also unzip the file and drop the .apk into system/app and the .so into system/lib.

AT&T

App version: 2.3.4

Features: Long press on menu button brings up search. Endless scrolling through home screens. Option to set app drawer alphabetically.

Epic 4G Touch (Sprint)

App version: 2.3.4

Features: Option to set app drawer alphabetically.

T-Mobile Galaxy S2 (Hercules)

App version: 2.3.5

Features: Long press on menu button brings up search. Endless scrolling through homescreens and widget selection screen. Option to set app drawer alphabetically.

credits: Intratech, x-cimo