Have you got the new thermostat?
It should have come with about 4 different gaskets, with varying diameters and thicknesses.
Drain your coolant
pop the top rad hose off this part:
Which will give you access to this black part.
This will have 3x bolts in which you need to remove and that will allow you to pop the black part off the thermo-housing. (the right one, the left is off a jp4)
Then with that off you should be able to see the thermostat inside, just pull it out. i suppose you could even use the origianl rubber ring/gasket on your new one.
Then just pop it all back together, re-fill with coolant, and bleed that