Nothing special. I used a card reader to partion a 1 GB SD card (/dev/sdc in my case) with cfdisk on Ubuntu Linux, like this:
1. FAT32 - 512 MB
2. Linux - 256 MB
3. Linux - 256 MB

Then I formatted partition 1 with
and partition 3 with
Then I pulled out the SD card and put it in the card reader again. The two formatted partitions where mounted automatically. I became root and changed directory to the third partiton. There I extracted the rootfs. Next I unmounted the partitions and put the SD card back into my N800 (internal slot next to the battery).

I connected the switched off N800 with the USB cable to the computer and started the flasher as root:
That's all.