You can make the mozzarella skulls a day in advance but the rest of the skewers only should take 5-10 minutes. If you make the skulls in advance keep them in an airtight container in the fridge. Also note, keep the faces up and not touching as they can get a bit deformed if they are squished together. However, you might also like as an extra effect when serving. I did notice that they were slightly tougher in texture since some of the water was released out of them in the oven.