As you remove the housing, pay attention to the direction of the thermostat. Clean the housing and engine opening of any old gasket material. Have a cloth or paper towel handy to clean up any pieces of the gasket that fall into the engine opening. Usually either the engine or the housing has a small grove in it to hold the thermostat in place. You will place the thermostat there.
Some manuals recommend adding sealer to the thermostat gasket for the best seal.
Install the gasket over the thermostat and housing, and place back on the car in reverse order. Be careful not to over-tighten the bolts, you can crack the housing or the engine mount. The best way to re-tighten is with use of a torque wrench. You can now re-connect the hose and refill the radiator. After your first refill, you may need to add more after the engine cools down.