Before you begin, ensure your PBO is actually 'bricked'. If you see the "Cannot Boot From HDD" message, your box isn't bricked - just be patient and it will eventually pick up the firmware from the USB drive. If all you see when you turn your PBO on is the Box Office Logo (with possible infinite reboot cycle), this should solve the problem:
If you don't have soldering wire, or something similar, take a twist-tie and strip the paper off to expose the wire.
Turn the switch on the back of the PBO to "OFF". Open up the PBO using these two screws, and slide out the board.
Orient the PBO's board as such. You will be inserting the wire into hole "A", while hole "B" is where you will be using your eyes to look for SW3. Plug a flash drive with firmware (preferably P02) into the front USB port. Ensure the power cord is plugged in.
Locate SW3; a flashlight may help
Turn the switch on the back of the PBO to "POWER", while ensuring:
A) The wire is touching the metal casing, in order to ground the switch.
B) The wire is also touching the SW3 solder point farthest to you, AND ONLY that point!
The firmware should start loading in 5-15 seconds. When the firmware starts installing (USB starts flashing, if you have a light on your USB drive), pull the wire out as quickly as you can, without touching any other areas on the board.