iirc, for a 400k floppy (not as reliable as with an 800k.400k one)

take a 1.4 meg floppy and tape over the hole without the slidey thing in it.

stick it into an old mac (iirc, system 6 or earlier, use the oldest OS you can)

when it prompts you to initillize it, choose single-sided.

write the 400k boot disk image to it.

done.