pretty easy mate, no need to drain the coolant but you will loose some so top up after, disconnect the bottom hose off rad at thermostat and support it so the coolant doesnt come out of rad, some coolant will come out.. take off thermostat housing, again some coolant will come out and replace old with new.. if you have not got the thermostat yet, get a 4efe corolla one as these are cheaper but they are the exact same..