Command-Option-Shift-Delete : Forces your Mac to startup from its internal CD-ROM drive or an external hard drive. Very helpful if you have a 3rd party CD-ROM drive that is not an Apple ROM device.
See also this.
The main issue is this. See here. If you have problems booting, see the section called “Troubleshooting the Installation Process”.
Currently, the only PowerPC subarchitectures that support CD-ROM booting are PReP and New World PowerMacs. On PowerMacs, hold the c key, or else the combination of Command, Option, Shift, and Delete keys together while booting to boot from the CD-ROM.
If your system doesn't boot directly from CD-ROM, you can still use the CD-ROM to install the system. On NewWorlds, you can also use an OpenFirmware command to boot from the CD-ROM manually. Follow the instructions in the section called “Booting NewWorld Macs from OpenFirmware” for booting from the hard disk, except use the path to yaboot on the CD at the OF prompt, such as
0 > boot cd:,\install\yaboot