Ok, some of these guides are confusing and I didn't explain very well, here you go.
This One is the simplest to explain
Follow that but note this:
Rename the bios file to this f12b.rom - for ease
The last step on page 2 disregarde their dos command
nvflash -p -u -f xxxxxxxx.rom as they made a mistake using switches no longer supported.
Use this
nvflash f12b.rom
Basically you just creating a boot disk, copying over the bios and nvflash
Then booting and flashing with that bios.