- format a SD card with FAT32
- extract the .NBH file from the rom image you like
- rename the .NBH to HERMIMG.NBH and copy it to your SD card
- put SD card in phone and start it in bootloader (hold OK button on left side and POWER button at same time and stick with stylus in RESET on bottom)
- confirm flashing on screen
- flashing might hang after some time but this is normal. Just wait. Complete flash takes about 30 minutes.
This also works for normal (non preproduction) devices but keep in mind that if the CID does not match the flashing might not work.
P.S. Same applies to other devices: BREEIMG.NBH, STARIMG.NBH, ...