1. Start with an empty (unpartitioned, unformatted) SD card. If you disk is not empty, you will need to do a search on how to clean the disk and create an msdos disklabel (aka partition table).

    Gnome Disks showing an empty, unpartitioned SD card

  2. Add boot partition

    Gnome Disks while adding a primary partition

  3. Format as VFAT (part of partition-creation wizard)

    Gnome Disks add partition wizard format as vfat

  4. Result is a partition formatted as FAT 16

    Gnome Disks showing SD card with FAT 16 boot partition

  5. Make the partition bootable

    Gnome Disks setting partition as bootable

  6. We want FAT 32 so use terminal to reformat as FAT 32

    Drop to terminal to reformat as FAT 32 because GNOME disks doesn’t have that option

  7. Resulting FAT 32 formatted boot partition

    Gnome Disks showing boot partition formatted as FAT 32

  8. Mount the boot partition for next steps

    Gnome Disks showing boot partition mounted