Here's the location of the drain plugs on your engine. Once you are under the car, they aren't that difficult to locate, but can be a b***h to get started if they have been in there since new. Be sure to use the right size 1/2" drive socket and some extensions.
I read lots of threads on changing coolant before I decided that draining the radiator and the block was more than adequate to switch out the old green to Zerex.
For some reason though, I was unable to get the final liter of Zerex in the plastic fill tank, until a about month later (summertime) when the level dropped enough to accomodate it. I assumed that there may have been some air in the system, possibly in the heater core, even though I did refill with the heater auto control on max defrost setting.
BTW, the design of the cooling system is interesting, in that it allows air entrapped in the coolant to percolate out through the small vent hose on the radiator top and from the thermostat housing into the upper portion of the plastic resevoir tank, under pressure. It's not your standard overflow tank/setup, since if the cap pressure is exceeded the coolant will spill out the overflow hose onto the ground.

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1986 560SL
2002 Toyota Camry
1993 Lexus