I want to change my car's coolant soon, but I'm not sure if I know what I'm doing because the car requires a special procedure.

When I had my Grand Prix, it required a special procedure too, and when I was changing its coolant, I accidently dropped the small screw/plug that goes into the engine, replaced it with another screw from my toolbox, and the car got extremely hot on the highway the day after, and almost overheated. Needless to say, I screwed that up bad.

I know I'll need DexCool, but is it fine to get Prestone's "DexCool approved" formula from Wal-Mart?

Bottom line, I'd like to know the exact procedure for changing my car's coolant because I don't want to mess up this engine.