Problems installing firmware update

I was notified by software update that I needed to install a firmware update for my G5, running X.3.5. I followed the instructions and downloaded the update then hit the shutdown button that appeared on the screen. I was told to hold the power button until I hear a long tone or the power button starts flashing then the update will install automatically. Well the problem is that after I click shutdown, and wait for the computer to shut off, I hold down the power button and the OS loads instead of installing the firmware update. I don't know what to do to make the update install. Your help is appreciated.