open the heater valve
remove the thermostat,reattach hose.
remove the top rad hose from rad,point it at one of your unsuspecting mates
then turn the motor on & pour new coolant in the bottom hose as fast as it'll drink it.
when ur mate is covered in about 2 litres of dirty coolant,switch off the motor.

nah,dont listen to me,i just remove the drain bolt......
easy when u got the right tool