Use volume and power buttons to choose YES (twice) - and you should see a TWRP screen (after a while) Don't worry - if you made everything correctly it should not wipe your phone. You should see a screen which is asking you if you want to factory reset your phone.
When you see LG logo release the POWER button (while still holding VOL DOWN) and press POWER button again. Now (put the battery in) and press POWER + VOL DOWN buttons. You can do that by holding power button or if that is not working take the battery out. When still in fastboot from step 3 you need to turn off your phone completely.
As on Marshmallow some fastboot commands are not working and LG is restoring recovery on every boot (so you cant use 'adb reboot recovery' right now) you need to use one trick.
