hi,
check out the thread below
https://forum.xda-developers.com/lg-g3/general/guide-root-lg-firmwares-kitkat-lollipop-t3056951
But don't forget the below steps if you are stuck at 90%.
1. Unplug phone from PC
2. Go to settings
3. Developer Options
4. Debugging -> Verify apps over USB
5...