If you use packaged polenta, slice it into 18 thin slices. If you make your own polenta, spread the cooked polenta out on a baking sheet and use a spoon or your moistened hands to spread it no thicker than 1/4 inch. When it has solidified, either cut it in half and use each half as one layer or cut it into an even number of pieces that you can fit together to fit your baking dish.