Have you installed the LG drivers and the Java sdk?
If so, boot into recovery by holding down home_key+volume_down+power for about 15 seconds. Don't worry if you don't see anything on the phone's screen. Plug your phone into your pc's usb port. Open a command prompt and cd into the android sdk platform-tools folder. Type
adb shell. This should connect you to your phone if you have everything installed and setup properly. If so, continue:
Type
exit to get back to your normal command prompt.
Download this recovery file and put it in the platform-tools directory.
Back in the command terminal, type:
adb push VM670NH_recovery.img /sdcard/VM670NH_recovery.img
Once it's done (takes a bit), type:
adb shell
This should give you a prompt with a # sign. If it's a $ sign, you aren't rooted, stop now, if it's a # sign, type:
flash_image recovery /sdcard/VM670NH_recovery.img
It should flash your new recovery in less than 30 seconds. If you get an error, stop.
If it flashed without error messages, type
exit, then unplug your phone, and pull out the battery. Put the battery back in, and boot into recovery again (holding down home_key+volume_key+power). This time, if all went well and the drivers were the issue, you should see the recovery menu after 30 seconds to a minute. Now you can hook your phone up to your computer and use the recovery menu to mount the sdcard, drag and drop a ROM zip file onto it, and be sure you have a copy of gapps zip file as well.
Once everything is on there, right click on the sdcard on your computer and choose eject, then unmount it on the phone. Next, wipe cache and under advanced, wipe dalvik cache. Next, format boot, system, data, and cache.
Now your phone is wiped and ready to install your new ROM. Choose install zip from sdcard,
DO NOT CHOOSE UPDATE! Find the ROM zip file and flash it. Repeat with the gapps zip file. Once both have been flashed, go back to the main recovery menu and select reboot.
Your phone will take longer than normal to reboot, be patient (up to 4-5 minutes first time).
If you have a problem at any step along the way, let us know and we should be able to help you out.