This recipe seems to work:
https://xilinx-wiki.atlassian.net/wiki/spaces/A/pages/18841655/Prepare+Boot+Medium

My version:

Find the device name.  For my Lenovo laptop with built-in card reader, it's {{{/dev/mmcblk0}}}.
Partitions are {{{/dev/mmcblk0p1}}} etc.  Unmount the card if mounted.

'''Erase the first sector'''

{{{
  $ dd if=/dev/zero of=/dev/xxxx bs=1024 count=1
}}}

'''Calculate {{{new_cylinders}}}

{{{
  $ fdisk -l /dev/xxxx

  Disk /dev/sdb: 8068 MB, 8068792320 bytes
  249 heads, 62 sectors/track, 1020 cylinders, total 15759360 sectors
  Units = sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disk identifier: 0x00000000
  
  Disk /dev/sdb doesn't contain a valid partition table
}}}

Note the size in bytes (i.e. 8068792320).  Calculate:

{{{new_cylinders = size_in_bytes/8225280}}}

(round down).  For this 8Gb example the result is 980.

'''Partition the card'''

Creat one 200MB FAT32 partition and the remainder as ext2
(ext4 should work too).  First, have to set CHS correctly.

{{{
  $ fdisk /dev/xxxx
}}}

(delete any existing partitions; shouldn't be any)

{{{
  Command (m for help): x
  Expert command (m for help): h
  Number of heads (1-256, default 30): 255
  Expert command (m for help): s
  Number of sectors (1-63, default 29): 63
  Expert command (m for help): c
  Number of cylinders (1-1048576, default 2286): <new_cylinders calculated from above>
  Expert command (m for help): r
}}}

Now create partitions:

{{{
Command (m for help): n
Partition type:
 p primary (0 primary, 0 extended, 4 free)
 e extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-15759359, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-15759359, default 15759359): +200M
  
Command (m for help): n
Partition type:
 p primary (1 primary, 0 extended, 3 free)
 e extended
Select (default p): p
Partition number (1-4, default 2): 2
First sector (411648-15759359, default 411648):
Using default value 411648
Last sector, +sectors or +size{K,M,G} (411648-15759359, default 15759359):
Using default value 15759359
}}}

Set bootable flag, partition types:

{{{
Command (m for help): a
Partition number (1-4): 1
  
Command (m for help): t
Partition number (1-4): 1
Hex code (type L to list codes): c
Changed system type of partition 1 to c (W95 FAT32 (LBA))
  
Command (m for help): t
Partition number (1-4): 2
Hex code (type L to list codes): 83
}}}

Check table and write changes:

{{{
Command (m for help): p
  
Disk /dev/sdb: 8068 MB, 8068792320 bytes
249 heads, 62 sectors/track, 1020 cylinders, total 15759360 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x920c958b
  
 Device Boot Start End Blocks Id System
/dev/sdb1 * 2048 411647 204800 c W95 FAT32 (LBA)
/dev/sdb2 411648 15759359 7673856 83 Linux
  
Command (m for help): w
The partition table has been altered!
  
Calling ioctl() to re-read partition table.
  
WARNING: If you have created or modified any DOS 6.x
partitions, please see the fdisk manual page for additional
information.
Syncing disks.
}}}

'''Create filesystems'''

{{{
mkfs.vfat -F 32 -n boot /dev/sdX1
mkfs.ext4 -L root /dev/sdX2
}}}

'''Mount boot partition'''

{{{
mkdir -p /mnt/boot
mount /dev/sdX1 /mnt/boot
}}}

'''Copy required files'''
{{{
cp BOOT.BIN /mnt/boot
cp image.ub /mnt/boot
umount /mnt/boot
}}}