Make sure you have the ROM zip on the SD card. Turn power off. Hold home+volume down+power. When you see the LG logo you can let go.
Once in your recovery, wipe cache, wipe dalvik cache, wipe data/factory reset. Go to mounts and storage. Format boot, system, data, and cache.
Go to install zip from SD card. Select zip from SD card. Select the ROM zip. Once it's installed if it needs the gapps, then select zip from SD card again. Select the gapps.
After all installing is complete go back to mounts and storage. Select format SD card(this will wipe all data from the SD card). Main menu, reboot system now.
Make sure you reboot the system a couple of times. Then go back into your recovery and make a backup of that ROM so the buyer is set up properly.