Enable Developer Options on LG phone

From your phone screen, tap to open Apps, then Settings app, scroll all the way down to About phone. Then find and open the Software information screen from which you can find the Android version, baseband version, software version, Kernel version, build number, etc. Tap Build number seven times in quick succession, you will get a notification saying you are now a developer. Now you can go back to the main Settings screen and find the new entry for Developer options there.

enable developer options lg g3

Enable USB Debugging on LG phone

Go to Apps >> Settings >> Developer options. Switch on Developer option, then tap the checkbox besides USB debugging to enable or disable it. When you are warned about the potential risk of allowing USB debugging, tap OK to confirm.