Hi,
I replaced the thermostat in my car about a month ago. Before you put it in, scrape out any crud from the groove on the manifold that it sits in. The top half of the manifold housing has a groove that holds the o-ring gasket, make sure you remove that... or when you bolt it on you'll snap your housing. Get some fine sandpaper (600-800 grit) on a flat surface and rub the flat side of the thermostat housing to make a nice clean surface.
Then it's just a matter of putting in the thermostat, make sure it sits flat, if you have a bit of contact glue, put a couple of drops on the thermostat housing to hold the gasket in the groove then place it on and do up the bolts. Do up the bolts gradually on each side to keep it sitting nice and flat... and don't overtighten them... or the housing will crack/snap.