It shouldn't come as too great a shock to find out that the thermostat on your VW is one of the more crucial parts when it comes to monitoring and regulating the internal temperature of your engine. It is essentially a heat-triggered valve that allows coolant from your radiator to flow into your engine once a certain operating temperature is reached. The hotter your engine gets, the wider the thermostat opens, and the more coolant gets pumped into your engine to cool it back down. Because of wear and tear--and because it has liquids flowing around it almost constantly--thermostats will have to be replaced periodically as they break down or become rusty, otherwise you risk overheating your car.
Locate your thermostat housing. This can be done by starting at the large hose connected to the top of your radiator and following it to where it connects to the engine. The housing on the other end of the hose can be removed, and you will find the thermostat underneath.
Use your wrench or your ratchet and socket to remove the thermostat bolts securing down the thermostat housing. Bolt sizes will vary depending on the model and year of the vehicle in question.
Pull back the thermostat housing to expose the thermostat. Note how the thermostat is oriented, and then remove it from the engine. Once you have the thermostat, you may discard it.
Use your razor blade or scraper tool to remove the remains of the thermostat gasket from the engine and from the housing.
Coat the bottom of the thermostat housing with a light layer of gasket sealant. Place the new gasket--it will have come with the thermostat when you purchased it--on the sealant, making sure that the gasket is oriented correctly to the housing.
Place the new thermostat into the engine block and orient it the way you noted in step 3.
Re-set the thermostat housing over the thermostat and screw the bolts back in to tighten it down.
