The thermostat is located below the throttle cables where the lower radiator hose goes in. What you need to do is remove the two bolts holding the water inlet in. Though you may want to drain most of your anti-freeze first. You can safe it and reuse it if you like.
Once you got the bolts off just force it off, it'll stick. The thermostat will probably stay attachen to the engine, pry it out. Place the new one in and rebolt it back up. The thermostat itself acts as a gasket, the rubber surround around the thermostat.
You may need to remove your throttle cables and mounting bracket for them to gain easier access to the water outlet.