Hi Manoj,
Here are some modification I propose for this doc. Hopes it helps 
There is a mistake in this LG v20 installation documentation (same in the upgrade doc)
At chapter “Booting a custom recovery using fastboot” under step 3:
This block :
You can also boot into fastboot mode via a key combination:
- With the device powered off
- hold Volume Up + Power.
Should be replaced with:
You can also boot into fastboot mode via a key combination:
- With the device powered off
- Prepare a USB-C cable, plug it only to the PC
- hold Volume Down + Plug the other extremity of the cable to the smartphone
Ref: https://www.hardreset.info/devices/lg/lg-v20/fastboot-mode/
Another info you can under chapter “Booting a custom recovery using fastboot” is that :
- the newest TWRP twrp-3.6.0_9-0-h990.img is working with Android R version (https://eu.dl.twrp.me/h990/twrp-3.6.0_9-0-h990.img.html)
Finally under chapter “Requirements” add this warning:
- After unlocking the bootloader when TWRP installed, performing a full NANDroid backup is hightly recommended (partitions /boot + /system + /data + /system-image + /efs)