There are few steps you need to takes and are as below:
1) First off all power off mobile take battery out and put battery back after 10 seconds
2) Battery percentage should be morethan 30% charge.
3) Now press Volume Down and Power button
4) When you see LG logo quickly leave both button then quickly press Volume Down and Volume Up button
5) When the 'Factory data reset' screen displays
6) Use the volume keys to select yes, use the power key to confirm.
7) Re-confirm by using the volume keys to select and power key to confirm.
From @Fushanj's Team