Unless you're lucky enough to be able to crack loose that drain plain on the back of the engine block on your own, which I couldn't do, you'll need to crank and run the car for just a few minutes to make sure you do get most of the fluid out, etc. And yes, there is a petcock at the bottom on the radiator that needs to be opened to AND you can disconnect the bottom hose to help drain things too.

If you can do it, it's best to run a detergent through for a couple of days before you do a fluid swap. Of course, you don't want to leave the detergent in for more than a couple or few days.