Not sure why you haven't watched some YouTube vids by actual Pros. Caulk the whole thing, as comfortably as you can. Use a spatula for cement to clean the areas. Then a dust brush then alcohol. Simple bathroom sealant should hold for many years. Make sure your layering technique is good so it doesn't come out ugly and it actually bonds. And get that shit in there, finger method works fine or you can buy that thingy they sell at the hardware stores that's a slider like tool.