[LG V20 Questions & Answers] GreenDinoXD:
Okay i am going to copy the exact text that the person who helped me said. It is really well explained and he says the steps very clearly. To do a TWRP backup and restore it it's one of the main buttons on the screen, I highly recommend using an SD card. Use the select storage button to change between internal storage and the SDcard, just check the box for data, and that has all your apps and pretty much everything else. And when you do the kdz part below you can just restore the version f firmware and do the update that way and skip the ota. I copy and pasted most of the rest from the last time I typed this up.

Get a hold of LGUP and downloading the kdz, heres (https://xdaforums.com/v20/how-to/v20-kdz-restore-failing-9-completing-fix-t3546817) a good thread on it, with even all the tools attached except the kdz, for getting on the older more stable twrp just flash flash it in fastboot (use lgup to restore the kdz which gets back to full stock, then use fastboot to get the info needed to fill out the form on LG's site(https://developer.lge.com/resource/mobile/RetrieveBootloader.dev), (basically this guide has it all listed out, plus once you've created an lg account and actually gotten to the form, the page with has directions too(http://www.droidviews.com/unlock-bootloader-on-lg-v20-us996/))

So heres how we're going to get twrp on here from a stock, but unlocked us996. You probably don't need the no-verity-opt-encrypt if also flashing supersu or magisk but I did it anyway to be safe.First get the latest version of no-verity-opt-encrypt.zip from here(https://build.nethunter.com/android-tools/no-verity-opt-encrypt/), and the stable version of Twrp-3.0.2-1-us996.img from a different directory on the same site(https://build.nethunter.com/test-builds/twrp/lge/), if you have an sd card in your phone put the no-verity zip on it ahead of time (and if you want to save a boot, put magisk on there too) (if not twrp should let you transfer it from your pc, but I havn't done it that way). So go ahead and reboot into fastboot (adb reboot bootloader) mode, then fastboot flash the_FULL_PATH_to_twrp-3.0.2-1-us996.img . once it successfully completes, until we flash the no verity zip in twrp if the phone boots into stock it will overwrite twrp and it will need to be reflashed in fastboot mode. So once you've flashed it the first time, pull the battery, wait a second, then put it back in, hold down volume down, hold the power button just until the LG logo appears, then let go of the power button, and immediately press (and release) it once while still holding down volume down, keep holding volume down until, if you've done it successfully, it will ask you to factory reset your phone (for future note, while twrp is installed this will NOT factory reset your phone), say yes both times and twrp will boot shortly.

From here, go to install, and then change to your sd card and install the no-verity-opt-encrypt.zip, then go back and hit wipe (in twrp), on the wipe screen, hit the format data button (the correct button will make you type out yes to confirm), from here twrp will be installed and not tampered with and you can safely install magisk or a custom rom, and/or boot back into stock.

I do not know where i downloaded magisk from but here is the latest version:https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445

I recommend getting version 13.3 and then upgrading to version 14. Hope this helps and if you have any questions feel free to ask, im just busy so i may not answer quickly, working on college applications so i apologize in advance