Originally Posted by scotty305

The thermostat housing usually has coolant in it. If you want to make sure there's no spill, drain at least a couple of liters of coolant out of the radiator.

If you know the coolant is pretty new you can re-use it if you drain into a clean container. If you're not sure how old the coolant is you should replace it every 1-2 years on these cars. The aluminum and cast iron engine materials tend to rust. Don't use tap water, use distilled water and coolant. Or get 50/50 mix coolant, that seems more common than concentrated coolant at the parts stores near me.

If the thermostat housing you bought is completely stock it will have a nipple that feeds coolant out to the Air Separator Tank. If you don't have an AST you need to block that nipple or it will be a massive coolant leak. It's hard to see in the photo, but it looks like that nipple has been removed and welded shut in @fendamonky 's photo. Don't just use a vacuum cap from a parts store, those don't work well for pressure especially when hot coolant is involved. Easiest way is to find a short length of spare coolant hose, a bolt that is just the right size to fit inside it tightly, and a clamp to hold the coolant hose on the bolt. A better way is to remove the nipple and drill & tap the hole so it's threaded and can be sealed with a bolt or an NPT plug. If you know a welder that is a nice solution also.