I've done many, but not recently. You need to be very careful about the antifreeze. Don't let any critters get to even a few drops. Basically, get one of those Prestone (or similar) flush kits. As I recall, you put the "T" fitting into the heater return hose. Engine hot so the t-stat is open and heater control to full. May be early, though. What car, how many miles, etc.?