2001 Buick Century Do it yourself Kit

I want to flush my coolant with a do it yourself kit. I am worried about cutting the hose and causing worse problems.
I read that flushing the system with tap water will kil your car. Is this true? I want to cut the heating hose, and put in the T pipe and attach a garden hose that will flush the system. Any ideas or suggestions? Oh, and my coolant looks very dirty. Under the radiator cap it is sludge.