From the bottom of the radiator. You'll have to remove the aerodynamic plastic pieces held by 10mm hex screws that cover the bottom of the radiator. The plastic "thumbscrew"-style threaded plug faces directly down, but it's impossible to see or get to until those black plastic pieces are out of the way. The plug has to be completely removed to drain - it's not a petcock affair, so watch your eyes to avoid splashing toxic ethylene glycol coolant in them. Don't lose the o-ring sealing coolant. And, don't get on this plug with pliers or a wrench when reinstalling unless you'd like to buy a new one. (You might need to "help" it carefully to initially loosen it, though.)
