First you should drain the antifreez by opening the drain plug located by the lower radiator hose. Then take the upper radiator hose off. Then take the 2 bolts out that hold the housing on. Remove the housng and there is your thermastat. Accordind to VW they want you to put antifreeze on the o-ring I use RTV a little extra help sealing just don't put a lot on. The book says to put thermastat in housing and turn 1/4 turn to right. I'm not sure why but thats what it says. TIP If you close the drain plug now and put as much antifreeze in as you can makes it easyer to get air pockets out later. Mount your therrmostast housing back to block and tighten bolts. Again put as much antifreeze in that you can them put upper radiator hose back on. Again put as much antifreeze in that you can. Now come the hard part sometimes getting all the air out of the cooling system. Put the radiator cap on but loose run the engine till the cooling fan kicks on but keep checking the coolant level. WARNING ANTIFREEZE WILL GET ABOVE 200 DEGREES
both upper and lower radiator hoses should be hot and radiator full. If both hoses aren't hot you have an air pocket that you have to get out. To do this run the engine a couple minutes after the fan kicks on then turn engine off for 5 or 10 minutes and start over keep doing this till both hoses get hot and the air the fan is blowing is hot. Once this is doe yout ready to go. I hose off any antifreeze that got one car so if you see antifreeze under car you know something is leaking and not what was spilled.