Installing KitKat

This is the short version. If you're new to custom ROMs, see [Installing LegacyXperia for Dummies](Installing LegacyXperia for Dummies)

Watch out for the Partitions

LX 11 (build2 and later) changes the internal partition sizes (the /Cache partition is minimized, and later the cache is stored in /Data).

If you install LX11 for the first time:
to make sure you start with standard partitions, flash stock ICS 4.1.B.0.587 full .ftf (Arc uses Arc S .ftf, Neo uses Neo V .ftf).
Get it here http://forum.xda-developers.com/showthread.php?t=2023504
here http://talk.sonymobile.com/t5/Software-Updates/Android-4-0-4-Build-Number-4-1-B-587-4-1-B-0-431-Manual-Update/td-p/169494 or here http://forum.android.com.pl/f308/oprogramowanie-dla-sony-ericsson-xperiaa-arc-s-147007/

If you are sure you never changed partitions since you were running stock ICS, you may skip this step.

Note: Downgrading from CM12.x or CM13.0 from LegacyXperia is possible without reflashing stock: execute flashboot erase system and flashboot erase userdata just after flashing the kernel and later on in recovery wipe system, data and cache as described below.

Installation

1. backup (Titanium or Helium; not NANDroid/CWM or anything that stores entire partiton images)
2. get ROM and matching lite GApps here: http://legacyxperia.github.io
3. extract kernel (boot.img) from ROM archive
4. copy ROM and GApps to SD
5. flash kernel using fastboot
6. go to recovery
   • In case the device does not boot to recovery, reboot to fastboot again and run:

   fastboot erase system
   fastboot erase userdata
   

7. when you install LX11 for the first time, you must format (not wipe) 3 partitions.
   Go to the mounts and storage sub-menu and format /System /Data /Cache.
   This will wipe and resize your internal storage.
   You must also reformat when you go back from LX11 to Jellybean.
   If you already run LX11, you don't need to wipe anything
8. flash ROM
9. flash GApps
10. do NOT restore NANDroid backups that were made with another partition layout
11. Did you flash the kernel from the .zip file you just downloaded? Step 5 above? If not, you failed.

More info on Mike's OP: http://forum.xda-developers.com/showpost.php?p=47895928&postcount=1

If you want to try out ART (you don't want it on a daily driver), switch before you install GApps and apps. Otherwise, your /data partition will probably fill up with precompiled cache data, and Android no longer boot.

Most people run out of space when they install their apps; use link2sd to move apps to the SD card.