Head into your local Supercheap and buy Green Nulon coolant.
http://www.supercheapauto.co.nz/Product/Nulon-Long-Life-Anti-Freeze-Anti-Boil-Concentrate-6-Litre/384350 <----Link to Product
Your car should be that same as mine which means your need a total of 7.5L total fluid in the radiator so what you wanna do is have a 1:2 mixing ratio..
In simple terms for every Litre of coolant add 2 Litres of water/ Add 2.5L of coolant concentrate and 5L of water
In terms of a flush, all i used was the 10 minute flush by Rislone because the water was all brown and gross. Real good stuff and gets rid of all the crap.
http://www.supercheapauto.co.nz/Product/Rislone-Radiator-10-Minute-Flush-355mL/290448<----Link to Product
With this you add all of it in and top it up with just water and wait till you are at operating temperature, while its warming up give your heater hoses a bit of a squeeze just so you get the rust and dried crap off the inside of your hoses
Once its running nice and you have squeezed as much as you can then turn the motor off, Wait a few minutes, open your radiator cap SLOWLY then let it drain out. Once its finished add the coolant in and top it up with water, once it has reached the top leave the radiator cap off, start the car and bleed the coolant of all air, once thats sorted then chuck the cap back on and you are sorted. What i usually do is take it for a little drive with a bottle of water in the back just incase. Thats pretty much the jist of it anyway.
Any other questions feel free to ask