A good way to flush (engine cold and off) is to remove the heater inlet hose at the left rear of the block after the system is drained. Flushing into the nipple flushes the engine, into the hose flushes the heater core circuit. Leave the block and radiator drain valves open during this procedure. You can buy inexpensive clamps, valves, and hose at hardware stores to make up your own flush kit. The top of the radiator and engine outlet hose are tough to flush, but you can do these areas by removing the engine outlet hose from the radiator nipple.

You can also fill the system with water, start it up and bring it up to temp once or twice to hot flush the engine of the last residual old antifreeze, after you've completed engine off cold flushing.

I never recommend using any kind of commercial "flush" product - just potable water.

Duke