Hello there, the purpose of this guide is to help you get your Arc Updated, De-Branded, Rooted, Themed or to get a Custom ROM installed on it.
The process of tinkering with any Device can seem a like a daunting process for New members but it's really quite simple so long as you read EVERY WORD of a thread describing how to Modify your Phone you shouldn't go far wrong.
Sure you might screw up once or twice but you can just simply run the Flashtool to restore your Phone to a Factory condition using a Branded or World Generic .ftf file which can be found in the Development Section
If you have Un-Locked your Bootloader DO NOT UPDATE Over The Air (O.T.A) or YOU WILL BRICK your Device. If you have Bricked then see THIS thread with how to Restore your Phone.
[Tutorial] Unbrick bricked (unlocked bootloader) Phones
[How-to] Repair Damaged Bootloader + Trim Area [Red Led][Unknown Device]
PLEASE ALSO NOTE:
Using the FOTA Bricked thread (above) has been confirmed to work on Phones with Locked Boot Loaders too.
Debranding & The Flashtool
Put simply Debranding removes Operator/Carrier bloatware, which can speed up your Phone, and the Flashtool allows you to Flash ROMs that are in .ftf (FlashTool File) format, so if your Carrier is slow with releasing the latest Updates you can just simply Flash it, which is easier than using SEUS or PC Companion. If your Phone comes SimFree then it's already De-Branded but if it came from a Carrier, O2, Orange, 3 etc then it will come preloaded with their Apps, Settings etc that you may not want. To debrand you need the Flashtool and a ROM. This video by BlueChipJ explains the process of using it...
You can find the latest Firmware LT15i 4.1.B.0.587 Generic Global World (Android 4.0.4) to be used with the Flashtool HERE
UPDATE: Root 4.1.B.0.587
[ 4.1.B.0.587 ] How To Root Xperia™ Smartphone [ Arc / Arc S & Ray]
[ARC/ARC-S]Autoroot for unlocked bootloader - no flashing needed
When you root your phone, you gain “superuser” privileges to the Android operating system. When you are using a stock device, you have the equivalent of “guest” privileges. If you are familiar with any Linux operating system, you know that the superuser privilege allows you to gain access to administrative rights. This level of access gets you down to the nitty gritty of the OS with no restrictions, lets you make changes to the system, and run scripts that would normally be blocked with a standard user account.
To Root your Arc whether your Boot Loader is Locked or Unlocked, simply look here - [16/Nov][ROOTING/UNROOTING] Xperia 2011 Easy Rooting Toolkit [v3.0](zergRush Exploit) - for another Revolutionary step in the Arc World from the legend that is DoomLord.
Got a Mac? - Rooting Arc on Mac O SX
Bootloader & Sim Unlocking
PLEASE NOTE: The ONLY WAY to Sim Unlock your Phone is to PAY for it.
[Guide] Unlocking bootloader on operator branded phones via Omnius
FREE!! For Generic Branded Phones - Unlock Arc bootloader with testpoint and dont lose DRM
[Tutorial] Relock bootloader!
[TOOL][FASTBOOT] QuikIMG - A frontend for fastboot [0.2 beta]
The stock bootloader on most Android phones is set up to run a signature check on any update file you attempt to flash to your NAND internal memory. So if you were to take a stock Android phone purchased from a carrier, and try to install a custom ROM on it, you'd get nowhere fast. Even after rooting, you still need to get access to the bootloader. Basically, unlocking the bootloader lets you install unsigned ROMs, like CyanogenMod.
There are two ways to Unlock yours, the Standard method (For SimFree Phones) which is explained HERE, (PLEASE NOTE! - You loose DRM when using the official method but not with the TestPoint) but if your Phone came from a Carrier then chances are (99%) you won't be able to use that method. There is a simple method of checking if you can or not which you'll find HERE. Then there is the TestPoint method.
Contact Alejandrissimo or Jinx13 for details of the remote service which isn't free, but cheaper than anywhere else on the net, and when you take in to account you will also SIM Unlock and retain DRM it's worth it.
Fastboot & Custom ROMS
How to install Fastboot on Windows 8
[TUTORIAL][STEP-BY-STEP][TEXT EDITION]How to install a custom ROM on Xperia Arc/S
A custom ROM is a fully standalone version of the OS, including the kernel (which makes everything run), apps, services, etc - everything you need to operate the device, except it's customized by someone in some way.
So what does the "customized" part mean? Since Android is open source, developers are free to take stock ROMs, modify them, strip them of garbage, optimize them, add things, and pretty much do whatever their imagination and skills allow.
In the Development Section you will find a few Custom ROMs, some require you to unlock your Boot Loader and some don't, and is usually stated in the thread title if it's required or not. You can find another great Guide HERE that will show you how to use Fastboot which is the method used for installing Custom ROMs.
Got a Mac? - [HOW-TO] Using Fastboot On Mac OSX
ClockworkMod Recovery/Nandroid BackUp
UPDATE: Got a Locked Bootloader?, this is for you - [APP]/[RECOVERY]x-Parts [CWM Installer] [Version 7.0] 29/6/12
ClockworkMod – also known as Clockwork and CWM – is a custom recovery for Android phones and tablets that allows you to perform several advanced recovery, restoration, installation and maintenance operations on your Android device
To use the CMW on a Stock ROM you will need to have Un-Locked Bootloader and run the RecoveryArc.img from the Computer, but if you use a Custom Kernel such as DoomKernel, or a Custom ROM such as CyanogenMod it comes as Standard. You can find the RecoveryArc.img HERE
CyanogenMod - CM FreeXperia Project
CyanogenMod (pronounced sigh-AN-oh-jen-mod), is a customized, aftermarket firmware distribution for several Android devices (See above for supported devices & how to install CyanogenMod on said devices). Based on the Android Open Source Project, CyanogenMod is designed to increase performance and reliability over Android-based ROMs released by vendors and carriers such as Google, T-Mobile, HTC, etc. CyanogenMod also offers a variety of features & enhancements that are not currently found in these versions of Android.
You can find the instructions of how to install it on your Arc and find the Free Xperia Project FXP CM7 Thread HERE, FXP CM9 HERE and FXP CM10 HERE
There are a good few themes in the Theme Section, all of which have clear instructions on how to install them in their First post (O.P). To make your own however is going to take some reading. You'll need to know how to do things like Decompiling, editing XML files and helps if you know how to use a Program like Photoshop or other similar software like Gimp.
Check out AUDIMEN's thread - [REF] * NEW - "Arc/Arc S" Collection Themes and Universal archive recovery *
There are a few good Programs to help you get started with modding Apks...
Diamondback's Virtuous Ten Studio
Then there are some other tools to help you with basic theming too...
[TOOL] [Theme Creator] XThemer for Xperia Devices
DefaultThemesCustomizer for SONY ICS
Xperia GX Homecustomizer By Adrian DC & Vr5411
If you prefer to do things manually like I do then this - [Guide/Tools] APK Modding/Theming Tips (Update 02/08/12) - might be of interest to you. Also check out the main Android Theme Section for icons, wallpapers and pretty much anything Theming related.
So that's that, the information contained at the links above should be enough to assist you to get your Arc up to date, Un-Locked or Modded and as I said at the start, read EVERY WORD of an Original Post [OP] (first post in a thread) before attempting any modification, THEN READ IT AGAIN, if you run in to issues it's more than likely you missed a step somewhere so just retrace your steps.
All credits go to the creators of the threads I've linked to.