Yes, you are right - you do already have most of the parts to the answer of your question.
For clarity here's the whole procedure:
1. Make sure you have cloned your drive properly, using Carbon Copy Cloner or SuperDuper!*, and had set the clone to be bootable in the relevant utlity's preferences.
2. With the external drive still connected via FireWire, restart the PowerBook.
3. Upon hearing the startup chime, press and hold down the 'option'/alt key until you see the Startup Manager.
4. Select the icon of the drive you want to boot up from.
5. Click the 'right arrow' icon to boot up from the drive you have chosen.
*SD! has an option to automatically reboot from the external drive after the cloning process.
15" 1.25GHz/12" 1GHz PBs, 2xPPC Mac minis, 12" iBook G4, Mac OS X (10.4.7) Cube, 2xTAMs, iPod 4G & nano 2G, iPs, AEBS, AX