First, Drain and fill to spec. I am almost certain that the fill is 2 qts. ( that is what my fill is on my 1998 gs300 ). DOUBLE CHECK YOUR OWNERS MANUAL.
Tape off with plastic sheeting all the electronics and "Steam clean " after treated with a mild cleaner. Repeat if needed. Set yourself up with a variety of brushes/old towels/and the like to get the nooks and cranny's.