Unlock and install custom ROM on LG G6 H872

Lots of steps to get it done. I love it when everything is done. My LG G6 H872 abandoned by T-Mobile is now running Android 10 using Havoc OS 3.7 smoothly.

Generally followed the steps from https://forum.xda-developers.com/tmobile-g6/how-to/root-h872-to-including-11g-t3775518. For simplicity and memory, below is my steps in brief.

1. Download and install ADB platform on Windows computer.
2. Download and install LG G6 USB driver.
3. Plug in LG G6 via USB to ensure the connection is stable and USB driver loads correctly.
4. In phone Settings, tap Software Build six time to enable Developer Mode.
5. In phone Settings, get into Developer Mode to enable Developer Connection and Debugging
6. With phone plugged in USB, run ADB command adb devices to verify the phone is online.
7. Run ADB command adb reboot bootloader
8. When the phone is rebooted into Download Mode, enter command fastboot oem unlock. Command fastboot devices may not show the phone, but it works with oem unlock command. Finally adb reboot to reboot the phone. This completes bootloader unlock.
9. Hold Volume Up button, without touching power button, plug in phone to USB. It will get phone into download mode.
10. Download the special LGUP program. 
11. Run it detect the plugged in phone in Download Mode,
12. Download the image of another LG phone with laf
13. Flash LG G6 with the image for laf only. Be careful with laf flashing. As soon as  laf flashing step is over, pull the USB cable immediately. The process only take one or two seconds. So be careful to pull quickly.
14. Download the current stock ROM the phone is using.
15. Flash the ROM using LGUP to phone again but without laf.
16. Download FWULL and burn it into USB flash drive.
17. Boot computer using FWUL USB flash drive.
18. Make sure the computer has wire connection to Internet
19. Connect phone in Download Mode to FWUL computer.
20. Enter a few commands (search online) in FWUL to download and install TWRP recovery system onto laf partition. This completes TWRP installation.
21. Reboot computer into Windows for more ADB commands.
22. Hold Volume Up button and plugin phone to get into Download Mode. Wait a while, it will go into TWRP.
23. In TWRP, ignore password, do a factory reset.
24. Download custom ROM Havoc OS for LG G6 and GAPPS 10 for ARM micro. Copy them into ADB folder.
25. Put phone into Sideload mode,
26. Run ADB command to sideload custom ROM zip file and GAPPS zip file: adb sideload filename.zip.
27. Reboot when sideload is complete by command adb reboot.
28. This completes custom ROM installation. Enjoy.

Note: TWRP is only installed in laf partition, to get into TWRP in future, must plug in USB to computer while holding Volume Up key.