Please provide explicit instructions.
Start up your G3 iMac by pushing the power-on button while holding the T-key until you see a dark blue screen with a large, 'bouncing' yellow Firewire symbol. Done.
If you do that when it's hooked up to the iMac running Mac OS X, the G3 will show up as a hard disk on the Mac OS X desktop provided the OS X Finder is set to show hard disks (which I believe is the default).
PS, to shut down the iMac in target disk mode, push its power button again.
