Radiator drain plug on a 2005 s-1...

The radiator drain valve is a couple of inches above the bottom of the radiator, on the passenger side of the car.
Insert a 1/4" drive ratchet into the center of the drain valve and loosened it about a half-turn. The center of the valve moves outward as it turns, and then coolant flows out through a drain hose.
I placed a dishpan under the front of the car to catch the coolant that flowed from the drain hose.
When closing the radiator drain valve, you need to push it inward and then tighten it. When fully closed, there is a small tab on the off-white center part that engages a slot in the black outer part.
Note that complete filling of the cooling system may be impossible until the engine gets hot enough to cause the thermostat to open and push out any air that may be trapped in the upper radiator hose. Many vehicles have a thermostat that opens at 190 or 195 degrees Fahrenheit (87 to 90 degrees Celsius).
On some vehicles I will run the engine for a while and watch the coolant inside the radiator. When the thermostat is closed, the coolant just sits there. Once the thermostat opens I can usually see the coolant moving across the radiator. BUT... the slanted radiator cap on the GMC Jimmy allows coolant to spill out, so I just install the radiator cap to the first "click", which covers the opening but doesn't seal the system.
Often after filling the cooling system it's necessary to "burp" the cooling system to get rid of any pockets of air. This can be done by running the engine for a few hours (or even a few days) with the radiator cap turned to the "first notch" which simply holds the cap in place while not forming a tight seal. Turning the radiator cap to the "second notch" forms the tight seal needed for proper pressurized operation.
The Jimmy's, Blazer's and S10's didn't seem to need any special burping procedures... I just ran it until the thermostat opened, added coolant to the radiator, and then topped up the coolant in the reservoir.
Good luck and hope this helps. If your having a hard time with the drain plug, just unhook the bottom radiator from the radiator and drain it that way.