Install TWRP via download mode. My instructions here:
http://androidforums.com/threads/st...he-most-of-download-mode.944059/#post-7086984
You don't need to worry about the part about backing up stock recovery in my post (the bottom part), since you can pull the recovery image from the official update if you need it later.
You can find SuperR's TWRP 2.8.6.0 in this post:
http://androidforums.com/threads/lg...d-unmodified-4-4-2.922509/page-6#post-7023913
To get into recovery from the power-off state, hold volume down and power long enough until the boot logo screen goes away and the second screen appears. Apply the bootloop fix zip in TWRP. The first time you try to reboot in TWRP, it will ask you whether you want to root by installing SuperSU (unless you've done it already). I'd suggest not doing it the first time. Reboot into the system and make sure everything is okay. Go back to TWRP and make a backup of your system files and data. Then you can root or apply other fixes/hacks in TWRP if you wish. Good luck.