...things like Spriggans and Dwemer Centurions? That's pretty much it. How's it done?

No Centurians. Spheres and Spiders are possible. To display trophies like those you need to pick the trophy room for the north wing and build the display pedastals. Then go to a pedestal and click on it. It will give you a menu like you get at a smelter or cooking pot for the ingredients you need.