Fill the radiator up, leave the cap off and start the car. Let it warm up and turn your heater on, the water has to get hot enough to open the thermostat and allow water to flow through the heater core. The water level in the radiator should go down slightly. Top it off and cap it and shut the car off.
The car should not be running when you put water or antifreeze in the cool radiator. This is because it is risky and might lead to accidents. It is recommended that you should add the coolant first before running the engine. Find out more from http://www.ehow.com/how_105_check-add-radiator.html.