I just replaced the coolant in my IS200 today.
1. Jack the car up and remove engine underbody tray #1
2. After ensuring the coolant is not hot, remove the the radiator cap and loosen the drain plug at the bottom corner of the radiator.
3. Loosen the small hose connected to the radiator cap and swing it down to drain out the reservoir.
4. Locate the drain plug on the engine. This bit is very difficult to see and can only be accessed from beneath the car (hence why you need to remove that underbody tray!). The engine drain plug is situated to the right of the oil filter. Connect a 1m length of hose to the nipple of the plug. Using a ratchet wrench and a 10mm socket, loosen the bolt to let the coolant out.
5. Tighten all the drain plugs + loosened hose.
6. Refill with new coolant.
7. With the car on an incline (facing UPHILL), start the engine with the radiator cap removed to let out any air stuck in the system. Fasten the cap when that's finished and you're done!
Remember to keep an eye on the temperature gauge and any coolant leaks.