Removing the engine cover like that is likely to break it - lift at the back first, then the front.
As the header tank easily lifts out, it's easier to empty it by removing it than pumping it out.
You don't need to mess about removing part or all of the undertray - you can reach the 'petcock' from the top and there's a hole in the tray to allow the water to flow out. You might need to arrange a bit of a deflector to guide it through the hole rather than flowing all over the undertray.
If your coolant and header tank is clean, as it should be on well-maintained IS250s you don't need to flush with 'distilled' water. It will just dilute the coolant. (And BTW - it's best to buy readymixed coolant - that way you get the correct mix and there's no risk of tapwater contamination should you plan to use that rather than deionised water.) The description of warming up and circulating the distilled water/coolant is plain wrong - that is not how temperature control of coolant flow works.
No mention of the cylinder block drains!
Apart from that....a useful guide.......
I know this Lexus technical photo is odd, but there are two cylinder block drain plugs. One locate in passenger side, other in driver side half bottom of engine. Correct step would be 1) flush radiator bottom, 2) clean reservoir tank, 3) unplug top and bottom hoses that goes engine to radiator, 4) drain engine block valves on the photo (Step 3, I done this job for waterpump + thermostat operation, you may not needed). To find two engine block plugs, https://www.my.is/forums/f221/is250-engine-block-coolant-drain-plug-location-463785/
Coolant change
in Lexus IS 250 / Lexus IS 250C Club / Lexus IS 220D & IS 200D Club
Posted
I know this Lexus technical photo is odd, but there are two cylinder block drain plugs. One locate in passenger side, other in driver side half bottom of engine. Correct step would be 1) flush radiator bottom, 2) clean reservoir tank, 3) unplug top and bottom hoses that goes engine to radiator, 4) drain engine block valves on the photo (Step 3, I done this job for waterpump + thermostat operation, you may not needed). To find two engine block plugs, https://www.my.is/forums/f221/is250-engine-block-coolant-drain-plug-location-463785/