Flash official Sony"s 4.1.2 6.2.B.0.211 with flashtool.
Root using DoomLord method.
Install a recovery.
Reboot in recovery, Wipe Data, Cache and Dalvik Cache.
Flash the rom.
Flash the update according to your phone (lt28h or lt28i): http://forum.xda-developers.com/show...&postcount=225
Flash 6.2.B.0.211 kernel only with flashtool (see FAQ).