I've had good results with the petcock under the exhaust manifold. If you put a short piece of tubing on the end of the petcock and let this drain into a dishpan, you can get most of the coolant. If you pull the lower radiato hose, you get a messy flood.

Once you have drained the coolant, you can open the lower hose and take off the thermostat housing. I run the garden hose down into the thermostat opening to flush. Have the heater temperature control set to high heat to help drain and flush that as well.

Install the new thermostat with the jiggle valve at the top as mentioned. Use a new ring gasket. Pay attention to the way the old thermostat was installed so you won't try to put the new one in backwards. (Don't ask me how I know this)

Button it all up and add coolant at the overflow tank. Fill to the middle of the tank, warm up the engine until the thermostat opens (feel the upper radiator hose and when it gets hot, the thremostat is open). There may be some air pockets to work out so keep checking the coolant level for a few days, add as needed.

You can clean bugs out of the radiator by blowing compressed air or a water stream from the back of the radiator toward the front.