Coolant leaking but not sure where

I've got a 2002 Ford E-150 with the V-6. (4.2 0r 4.6?) It's been leaking coolant since last year. I tried to flush the coolant system but I may not have done it correctly. Are you supposed to run the heater with the engine idling after you've filled it water? Could you provide the corect order to flush your coolant? It still leaks so badly that it's empty in a couple days. Thanks in advance.