If you're doing a new install, you'll need to create a BIOS cd on your PC. There are good guides on xbox-scene.com, i.e. how to do it, and what programs to use. Once the chip boots to the FlashBIOS screen, you use the BIOS cd to flash the BIOS.
If you're Xbox is chipped and loads a dashboard, you can use FlashX1.2 (Xbox app) to flash the BIOS from the HD.