Coolant flush I asked a friend of mine.

Coolant flush
i asked a friend of mine, a mechanic, about doing a coolant flush at home, given I don't have the coolant flush machine.
He said: warm up the car. Disconnect upper radiator hose. Insert a garden hose in the disconnected hose (disconnected at the radiator, so the water from garden hose would flow into the block. He said I could also add a Flush Capsule(?) Sold at Parts Store). And when the coolant coming out of the radiator turned to water, to withdraw the garden hose, and let the engine running for about 1 minute and shut it off. He said at that time all the water would have come out of the block.
Now reconnect radiator hose and fill with fresh coolant.
i'm a liitle appehensive whether this method was safe even though it was for 1 minute. Even when the garden hose was inserted with water flowing, it would appear to me it was not safe, because it's just water ( couldn't this cause a crack( a fissure))?
is this method safe?
if not, what are your recommendation for the DIY?