You need: a multimeter for measuring, a hdmi breakout adapter, a molex 12v power supply and an old iMac G4.
If your iMac G4 is working I wouldn't do it - keep it running. I only did it because my power supply was broken.
Dumbing it down: Remove all components from your old iMac, strip the inverter and TMDS cable and attach the wires to the power source and HDMI port.
If you're completely inexperienced with electronics maybe watch some videos on youtube on simple electronics first.