I'll asume you are talking Linux, therefor:
NOTE: these instructions are a cut & paste from the Dennis Warner web site.
Copyright (c) 2002-2003 by Dennis Warner
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation.
The following command will create a Linux boot disk:
# mkbootdisk <kernel version number>
To check out what kernel version you are running, try:
$ uname -r
So, if you are running kernel 2.4.18-4,
the command would look like:
# mkbootdisk 2.4.18-4
You should then see:
Insert a disk in /dev/fd0. Any information on the disk will be lost.
Press <Enter> to continue or ^C to abort:
Note: ^C = Ctrl-C
After pressing <Enter> you should see your floppy drive light come on
and be active for a minute or so.
Once the floppy drive light goes off, mount your floppy:
# mount /dev/fd0 /mnt/floppy
and check out the disk:
# ls /mnt/floppy
You should see the following files:
boot.msg
initrd.img
ldlinux.sys
syslinux.cfg
vmlinuz
Now, try your newly made boot disk out. Make sure it is in the floppy drive and restart your system. When your system begins to restart, go into your BIOS setup. In there, you will need to set the floppy as the first boot drive.
! Make note of your current settings, so you can change them back after the test.
Once you see the boot disk works, label it and store the disk in a safe place. You never know when you will need it!
Relevant help files:
man mkbootdisk
man ls
man uname
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(Saves me typing)

Good Luck,
Laurie,