Drain Coolant, or place a receptacle under the vehicle to catch the coolant that will come out when you remove the thermostat housing.
Remove the two bolts that hold the thermostat housing in place. Be careful, alot of times the bolts will be corroded and if given much pressure, they'll snap off like a twig.
It's a real PITA when you gotta take everything apart to drill out a bolt you've broken off.
Clean the mounting surfaces real well with a bit of emery cloth, sand-paper or a stiff wire brush.
Some will tell you to use a gasket sealer or high-temp silicone or such to make sure you get a good seal. That's a matter of personal preference. I generally don't, as if the surfaces are cleaned real well, they don't typically leak.
Place new thermostat and gasket in place, ensuring the thermostat is placed in the block in the proper direction.
Place the thermostat housing back into place, and insert and tighten the two bolts that hold it there.
Refill your coolant levels, run engine, and check for leaks. You may have to add to the coolant levels after you run it for a while, as it will probably suck the level down as it gets warmer.
While you're at it, always good to check the condition of the belts and hoses.
I recently replaced a thermostat in a car of mine, only to have the newly found water pressure create problems with the water-pump, heater and radiator hoses. Had to tear it all apart again.
That wouldn't have been so bad, but I had to pull the radiator and grill a second time to get to it. Sure would have saved myself alot of frustration if I had just done it all the first time.
