Way back under the exhaust manifold you'll find a brass nipple extending out of the block. You can unscrew it with a box end wrench (13mm?) and that will drain some coolant out. If you use a hose on the nipple and a bucket you won't make a mess. This drops the level of the coolant down a bit so when you remove the thermostat housing and remove the t'stat from the housing you dont' get all messed up. If you don't draw down the level of coolant first, you'll get a geyser in your hands when you pluck the t'stat from its housing. There's no right or wrong about this; it's your choice. If you do get a geyser, rinse things down with a hose, and be careful about water/coolant getting into your distributor cap. (Ask me how I know about this...)

Get a good thermostat (no cheapies) and a rubber gasket. When you install the new one, make sure the little bubble/bleed hole in the thermostat body is oriented up, not down. This will enable air bubbles to bleed through up into the expansion tank.

Do you know anything about the history of your cooling system? If not, consider replacing the upper and lower radiator hoses at the same time, and consider draining the old coolant out and replacing with fresh (heat control valve slid over to hot for this).

Rob Kuhlman