[LG G6 Guides, News, & Discussion] challenger07:
There are a number of generic installation guides for custom roms eg https://www.xda-developers.com/how-to-install-custom-rom-android/

However this one is specifically for the LG G6 and tested using the US997 model. It worked for me, but I welcome comments on how to improve it.

Create a folder on the PC for all the downloads and files

Download phone driver from LG website

https://www.lg.com/us/support/help-library/lg-mobile-drivers-and-software-CT10000027-20150179827560

Download ADB/Fastboot https://developer.android.com/studio/releases/platform-tools (You need to unzip once downloaded)

(I had an error "adb server version (31) doesn't match this client (41) killing". I searched my PC and found an older version which I had used for another phone and deleted the folders and reinstalled from the download)

Ensure the phone is backing up. (Google backup is working, you have all your photos music etc downloaded) the next steps will wipe the phone

Remove the PIN/Password/Fingerprints from the phone so it will boot without any security.

Unlock the bootloader

https://developer.lge.com/resource/mobile/RetrieveBootloader.dev

The phone will then reboot and do a complete wipe so you need to wait a while for Android to setup.

Telegraph has the latest news on LG G6 ROMs and Recovery. From here I got the links for SHRP recovery which seems more up to date for LG G6 than TWRP. (https://web.telegram.org/#/im?p=@g6releases)

https://sourceforge.net/projects/pa-g6/files/Releases/SHRP/20.07.2020/US997/

I named the downloaded file "recovery" (You need the .img file not the ZIP)

There are lots of active ROMS for LG G6 I have Lineage installed on another phone which works really well, but Havoc seems the most popular on the G6.

Download the ROM and Gapps https://opengapps.org/ (I used Nano, remember G6 is 64bit)

You will also need to update the bootloader. This was the latest I found for the US997

https://androidfilehost.com/?fid=1899786940962575226

(I missed the bootloader the first time and Gapps will not flash, I was helped by the LG G6 community on Telegram https://web.telegram.org/#/im?p=@g6com)

Now you have downloaded all you need

Then connect the phone to the PC and ensured you have ADB debug on in developer options and boot the phone into fastboot

On the PC open a command prompt in the folder you have all the files:

adb reboot bootloader

To load the recovery to the phone :

Fastboot flash recovery recovery.img

YOU MUST NOW BOOT DIRECTLY TO RECOVERY. Do not reboot to Android as this will automatically reset recovery to the stock version.

Unplug the USB cable and reboot to recovery (hold volume down + power button).. continue holding until phone reboots...

As soon as you see the LG logo on the screen.. let go of the power button then quickly press it again (never letting go of volume down).

Keep holding until you see the Factory Reset screen. Click thru the factory reset screens using the volume buttons to move up/down and power button to select. Choose "Yes" twice (saying you want to reset the phone it should boot into your new recovery. Despite what the screen says it won't actually factory reset/wipe your data... as long as you've installed a custom recovery and have not booted into Android since then.

Most likely you will be asked the password to decrypt the data folder. I tried my phone PIN and other default passwords suggested on the internet. I eventually found that this is a known issue and you need for cancel out of this screen and go and "FORMAT DATA"

Reboot directly to recovery (select reboot recovery in Skyhawk), this time you should not be asked for the decryption password.

I recommend you backup your current setup before making changes. If you hit issues you can use the recovery to restore. My SD card slot is broken (Poor LG design) so I plugged a USB thumb drive into the USB C slot and did a full backup. (No data since I just formatted it).

Wipe the partitions before you flash

Full wipe ("The only option you ever need")

Select Advanced and wipe system partition.

FLASH (You need to put the ZIP files either on the USB drive as I did, or copy to your SD card, or you can use ADB sideloader from the PC to push the files to the phone). The USB drive seems the easiest since you can plug it into your PC to load the files then plug it into the phone to read them.

Flash in the following order

Bootloader

ROM

Gapps

After Installing wipe cache and Davik

Reboot

It will do lots of processing while setting up. Ensure it stays cool (remove case and leave it plugged in with the screen off)

Donate to the developer of your new ROM at the end when it works.