Plug xbox and computer into router... Make sure the duox2 is on the flashbios bank and turn them both on. Choose "Enable Network Flashing" from the main flashbios menu and wait for it to display an IP on screen.
Once the IP comes up, put that in a web browser on your PC. Once the page loads, use it to choose your 512k bios file and click flash. Then just wait for the xbox to shut off. Done!