For flashing ROM copy SSPL-TRIN.exe to your device and run it. Standart bootloader prompt should appear on screen, check SPL version number: if it ends with ".ds" then everything is fine. (Re)attach usb cable and run firmware update utility.
In case if something goes wrong during flashing you should flash your device with ROM accepted by standard bootloader (and have another try if you wish).
(*) code must be formally signed i.e. contain properly-formated signatures on their places, however no checking are performed against them. Actualy unsigned code flashing was tested by flashing original signed nbh image with few bytes in header and extrom section modified, but patches were designed to accept any properly formatted crap as a valid signature.
See readme.txt for more info
Русскоязычное обсуждение здесь
If you feel yourself as an advanced user and going to do things more complex than described above - read readme.txt before doing them !