To REALLY drain the block, you'd just have water go in the thermostat housing, and drain out of the block directly using the drain plug. There is a small drain plug on the block intended for this, but there's a good chance it's never been used and is rusted/seized in place.
The only reason to flush a system that way if it's incredibly rusted and contaminated, and nothing has worked to get clean, clear water when flushing every other way.
Generally you can get a system pretty clean just pouring water directly into the radiator, following the correct steps to "burp" the air, then running it for awhile like that, and repeating until you have drained and refilled several times.
The other option is the hose adapter, where you add a "T" fitting into the heater core line, attach a hose to the fitting, then drain the system, turn the hose on, and start the engine, letting the water from the hose go through the system and drain out.
Either/or gets the system pretty clean without having to do each part seperately. Personally I've taken to doing it the first way with jugs of distilled water, as tap water can leave sediment or residue in the system, and distilled water is usually "cleaner".
If the cooling system was poorly maintained or never serviced, and the coolant is rusty, brown, or worse, getting to that drain plug on the block and flushing it as much as you can, along with the radiator and heater core, will go a long way towards cleaning it up.
