ok, a little more detailed
1/ unmount the card
2/ do "fdisk -l" to list disks/cards, and then "fdisk /dev/sd$" to partition it where $ is the device
3/ delete all partitions, then
- create a new partition of type primary, use half the card size (start is default, end is (say) +1G). set the type to LBA FAT16.
- create a new partition primary, use rest of the card.
4/ write the partition table
5/ mkfs.vfat /dev/sd$1
6/ mkfs.ext2 /dev/sd$2
7/ eject the card and reinsert, and hopefully the two partitions will be mounted, or mount manually.
unpack the tar file to the ext2 directory, and I think you simply chroot into it?
--edit-- in instructions above I suggested a 1G fat partition so that the card will behave as a useful flash drive acceptable to windows etc to make it easy to transfer files, and also in the hope that inserting it into a windows or mac will not cause them to try and format the card
you could make that partition much smaller if wished.