Piss easy mate. Undo the bottom rad hose, let all the coolent drain out, put it back on. Then unscrew the bleed screw in the rad + undo one of the hoses that goes into the expansion tank and stick a hose through it untill the water comes out the bleed screw, stop when the water that comes out is clean. Finaly put the hose back on the tank (leave the bleed screw off) and put a 60/40 mix in, wait for it to pour out the bleed screw, so you dont get airlocks.
Also run the car for 5minutes with the cap off the expansion bottle