On some computers BIOS does not support booting to the CD-ROM drive. On other computers the BIOS can be set to boot to the CD-ROM first but it doesn't work.
First download sbm.img disk image.
Now you need to copy the file to a floppy disk as an image. Note to Windows users Drag and Drop or copy / paste will not do what you want.
Linux users run the dd command and optionally the compare command.
Insert floppy, CD and reboot. When the SBM menu comes up press down arrow until CD-ROM is highlighted and press enter. It may ask a question or two. It may even give you an error message or two. Just keep pressing enter until the computer start booting to the CD.
Did I mention there is a learning curve involved?