Replace the radiator drain plug or fully close the petcock.
Using a funnel to eliminate spills, fill the radiator with a 50/50 mixture of coolant and water. I'm a big fan of the premixed coolant that's become popular lately, it eliminates the measuring or guessing step. With the radiator filled, go ahead and fill the plastic coolant reservoir if your car has separate openings, again with a 50/50 mix.
Tighten all of your caps well and you're like Fonzarelli -- cool!
It's a good idea to check your radiator coolant level in a day or so to be sure it's proper, sometimes an air bubble works its way out and you need to add a little.