Download a program called FlashX. It was made by the Xecuter team.
Put it on your xbox like any other program and put the 256k bios in C:bios folder.
On running FlashX you pick the bios file from C:bios and then choose which part of the mod you want to flash it to.
No need to resize, the prog takes it into account automatically. Just pick bank 1 or bank 3. Picking bank 2 or bank 4 for a 256k bios is useless because the chip is only designed to launch from 1 or 3, the others are there for the x2pro.
Just follow the onscreen instructions. You can switch between 1 and 3 for booting using the switches provided (or a jumper for the old versions).