Open up the drain bolt and put garden hose in the opening where the upper radiator pipe connects. The water will go in through the block and drain through the block drain opening. No need to start the engine. But for the full citric acid flush, you need to remove the thermostat and run the mixture through the cooling system by starting the car. First asses the condition of rust and calcium/salt deposits. If it is something like mine, then you need a full flush. Otherwise, you just drain the block and radiator and flush it with plain water without removing thermostat.
