From ArchWiki

i18n
English
简体中文

First you need to install GRUB if you have not already done so:

 pacman -S grub

Next you will need to format a floppy disk:

 fdformat /dev/fd0
 mke2fs /dev/fd0

Now mount the disk:

 mount -t ext2 /dev/fd0 /mnt/fl

Install grub to the disk:

 grub-install --root-directory=/mnt/fl '(fd0)'

Copy your menu.lst file to the disk:

 cp /boot/grub/menu.lst /mnt/fl/boot/grub/menu.lst

Now unmount your floppy:

 umount /mnt/fl

You're done! Now you should be able to restart your computer with the disk in it and it should boot up to GRUB. Make sure that your floppy disk is set to have higher priority than your hard drive when booting in your BIOS first of course.

For an automated approach, see the Super GRUB Boot Disk