Reboot into Recovery
You first have to install Custom Recovery, The easiest way is to download Rom Manager and flash recovery from there (your phone should be rooted for that).
After the recovery is successfully flashed, you can reboot into recovery from there itself (there is an option for that).
Or if you want to manually reboot into recovery, shut down the phone and press and hold Power + Vol up + Home keys.
If it didn't work, again shut down the phone, turn on the phone and as soon as you see the LG logo, press and hold Vol UP/DOWN + Home keys (no need to press or hold the Power key).