it's not that bad. you need to find the wire diagram for your main and accessory harness for the gx470.
u need power, ground, ignition, brake pedal switch, immobilizer lines: +,-,data, turn signals, horn...maybe some others.
u should find year specific diagrams.
for the 2008...not sure what year she has
http://www.modifiedlife.com/2008-lexus-gx470-remote-start-wiring-diagram/