I had to do it once while almost stranded. I was just lucky enough to get us in to a gas station before the car over heated. I used towls to slowly let the pressure go down. I would open it slightly for 30 seconds then close it to let the coolent settle down. After ten minutes of doing that I finally left it alone for five minutes before just opening it up and having dirty coolent spray out as the air bubbled out of my radiator. I stepped back and avoided the blast.  I then poured a whole bottle of fresh pre-mixed coolent in. It took a while longer for the coolent to fully cycle through the system and bring the engine temp back down to normal.