I found it works well to use an already working Linux system and just do this:
dd if=boot-floppy-hfs.img of=/dev/fd0
If the floppy doesn't work, you may need to fdformat it first. Also, make sure it's not a 400K disk
I have a question now: I tried using the boot-floppy-hfs.img on my Power Macintosh 6100, and it showed a Linux logo in place of the standard happy face for about 20 seconds, but then it filled the screen with garbage and stopped running. What could have caused that?