The steps are outlined in the link Unforgiven already posted:
xda-developers - View Single Post - LG Optimus L7 (P700) Development
The only hangup you MAY run into is at steps 4 & 5 of that guide. They assume you have a booting phone and can turn on usb debugging and put the phone connection into PC Software mode. That may or may not be a problem especially if you've perhaps enabled usb debugging in the past as a root user. I'm not sure if the PC Software mode thing is absolutely required to move forward or not, you'll have to try it and see.
If it doesn't work, the next idea would be to try and instead use the LG Flash Tools software which should allow you to interface the phone to the PC and then load the kdz file. Either way, you'll need the appropriate kdz file as outlined in the linked thread.