the best way to do it is a complete flush

first drain the coolant through the plug at the bottom of the radiator... then fill up with water, tap water will do...

then start the engine.... wait for the radiator fan to turn on and when fan stops, unplug the drain plug again to let out water and fill radiator up with water. this is becoz when ur fan is on that means the water from the radiator is circulated to the engine and vice versa....
do this about 2-3 times to ensure all coolant form the engine and radiator is compleatly flushed...

after that pre mix ur coolant (with water, ususally distilled) in a CLEAN bucket according to the instructions on the coolant bottle (b4 this see the owners manual to determine how much coolant ur car needs)...

with this pre mixed coolant fill up the radiator when u drain out the water for the 3rd time (if u do three times la). then wait for the fan to on, then when if goes off, drain the water and put in the coolant. repeat this process till ur pre mix coolant in the bucket finish...... only close the radiator cap when the fan is on.... u may add a bit of distilled water to top it up...

and also dont forget to put some pre mix into ur resovoir

careful ya, its hot water.... and pls pre mix it first in a bucket before u start anything.... dont let the fan run without anything in the radiator... or else all there will be in the water jackets is air pockets..

good luck

distilled water is PURE H2O... while tap water MAY CONTAIN other contaminants that may cause sediments in the long run...
if flush then nvm la