You may want to consider adding a cooling system flush cleaner and run the engine for a bit before beginning the flush.  It will do a bit better job.

 

A flush tee can be installed in the heater hose, or you can disconnect a heater hose.  Remove the radiator cap.  Connect your garden hose to the tee, or one side of the disconnected hose.  Let the water run until the water coming out of the radiator is clean.  If you are using a tee, pinch the hose on one side of the tee, and run until clean.  Then pinch the other side of the hose and run until clean.  Go back and forth several times.   When you switch directions it may take a minute to get dirty before it cleans up again.

If you are disconnecting the hose, put your garden hose to the heater hose, then switch back and forth to the port that you took the hose off of.  Keep going back and forth as if the tee was there.

 

If you do not alternate the flow back and forth, you won't break loose all the crud and get a good cleaning.    Any flush machine can do no more than a good rinse combined with the pulse, back and forth motion.

 

 

When you are convinced it is clean, shut off the water, drain the radiator, and add the correct amount of antifreeze.