I have a 1992 Chevy caprice how do I flush the radiator

If you are having issues with the heater blowing cold, flushing the radiator is not likely to help. There are two reasons that the heater blows cold.
1. The heater controls for the blend door are not directing the blower air thru the heater core. This is the most common problem.
2. The heater core is clogged and not passing coolant through it.

Flushing the radiator in most cases will not clear a clogged heater core as the core is supplied with its coolant as a separate loop. Flush water will simply follow the path of least resistance thru the engine block and not flow through the core. You CAN try to flush the core by removing the heater hoses from the cooling system and backflushing the core in an attempt to remove whatever is clogging it. Be careful not to apply to much pressure to the core as some cores are plastic-bodied and could burst if overpressured. You can backflush by attaching a garden hose to the output of the core and slowly opening the hose valve. Ensure that the other side of the core is disconnected from the car and can allow the debris to flow out. This does not always work.

Remove the lower hose or hoses from the radiator. Put a big tub under the radiator to catch all the fluid. Take off the radiator cap, get a hose, turn on the car, turn on the hose and pour the water in the radiator for a minute or until your tub is almost full or you don't see colored fluid coming out. Turn off the car and let the rest of the water drain out. Replace the lower hose(s), fill the radiator, empty the radiator over flow reservoir and refill with new fluid 70/30 mix of fluid to water.

Remove hoses and use garden hose to run water through both tubes then reverse the flow and flush some more. If that won't clear the debris then replacement will be necessary