The block drain plug on 1zz is at lower edge of block facing back of car, its a 12mm head steel plug in a steel casing, it wont snap or break.
However, the best flush is done by draining the radiator first fully (it takes 20 minutes on corolla 120) - then also opening the block drain.
find a heater hose that goes into engine, push water into it with your garden hose to flush everything out, the water will go everywhere and behind the thermostat to drain everything.
close all taps - fill with some RO water and start engine for 2 minutes, then shut it off and open drain taps again - wait another 20 minutes for it to drain, close them up.
now measure the expansion tank volume separately - this is also part of the total capacity mentioned by toyota in the service manual, so when adding concentrate to engine you need to adjust the volume (its a very small value though)
fill up the system with your calculated concentrate volume slowly then add water on top - start engine and keep on topping for about 5 minutes - run heater and check for hot air to make sure all air pockets are removed from heater too.
Point of experience: This toyota will not shoot hot air from the heater until you cap the radiator. its due to the way the system is plumbed. So check for both pipes to get hot - then cap radiator and check heater.
I would suggest to install brand new radiator cap too.
