For those of you who want to install custom ROMs on your LG G3, first you will want to learn how to make a backup ROM.
In this tutorial, we will show you how to backup and restore ROM on your rooted LG G3 using TWRP Recovery.
Backing up a ROM will backup your whole ROM (including kernel) along with your Apps, App Data and pretty much everything on your phone except personal files like photos, videos, etc…etc…
If you have lots of games on your phone, your backup may be larger than usual but a backup ROM usually is around 2GB.
I highly recommend using an OTG USB Flash Drive, this will keep all of your backup ROMs separate from your internal storage/micro-SD card.
First, you will obviously need a rooted LG G3 with TWRP Recovery installed. (At the time of writing this, TWRP recovery is only available for T-Mobile G3 but should be available for other variants soon.)
Step 1. Use Quick Boot app to reboot into your TWRP recovery.
Step 2. Choose “Backup” once in TWRP Recovery.
Step 3. Choose “Storage”.
Step 4. You can backup your ROM to your internal storage, micro SD card, or even OTG USB Flash drive.
Step 5. Next choose “Backup Name” if you want to set a custom backup name.
Step 6. Swipe to start backing up!
In my case, I had a couple games that were near 1 gig each so my total backup size became around 5.7GB.