Preheat oven to 400F. If you are using instant lasagne sheets, soak them for 5 seconds in a bowl of hot water. As they soften, remove them from the bowl and pat them dry (if you leave them in the water too long they will stick together). Spoon 1/2 cup of the spaghetti sauce in the bottom of an 8-inch square baking pan, and top with a sheet of "instant" lasagne or 2 cups of cooked noodles. Spread the pasta with dabs of ricotta and some of the sauce. Be sure to spread the cheese and sauce all over the lasagne or noodles. Sprinkle with some parmesan cheese. Repeat the layering until you have used all the ingredients, ending with a sheet of lasagne, completely covered with the sauce (even the corners). If using cooked noodles, make only 2 layers of noodles. Bake, covered, for 30 minutes. Then uncover and bake for 5 minutes. Remove pan from the oven and let the lasagne set for 5 minutes before slicing and serving.