It previously there was coolent then just drain the coolant from drain bolt at radiator....
And put a bit more then the half of the amount of concentrated coolant that come out of radiator....
Then top it with the water ( not tap water)
I will not suggest to go for complete flush as previously it has coolant so no need for extra hassle...
For complete flush drain radiator then you must remove thermostat valve and radiator pipe pump air it the radiator pipe such that the remaining coolant come out of head and pipes.... then remove heater core pipes and pump air from one pipe coolant will come out of another one... you have flush the cooling system first attach radiator draining bolt, then put radiator pipes.... attach the in-line of heater core... let other remain open.... put coolant (diluted or pre mix) jn radiator slow remember slowly till it come to top...
Don't close the radiator cap...
Then hold some pot at the open pipe of heater core and put other hand on the point where it needs to be connected (heater core line)
Ask someone to start the car...after 1 min approx coolent will be at the bottom of radiator ask someone to keep filling it ... let it remain on till you see coolant (without bubbles) coming out of heater cores pipe once it is without bubbles turn of the car and attach the pipe to the point where it belongs.... make sure to remove your hand from that port before attach the pipe
...
Start the car... let it wait with radiator cap removed for 5 min... top up if required.... turn it off place back the cap... put coolant in expansion bottle... let the car cool down.... drive like that for 1 day or 2.... keep checking coolant level.... if reduce top it up... and when it stops reducing coolant open thermostat housing place thermostat.... replace gasket/ seal close it and you are good to go.....
