Follow these detailed steps to create your stuffed shells masterpiece:

1. Cook the Pasta: Boil a pot of salted water and cook the pasta shells according to the package instructions. Drain and set aside.
2. Preheat Oven: Preheat your oven to 375°F (190°C).
3. Prepare the Filling: In a large bowl, combine the ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, egg, spinach (if using), garlic powder, Italian seasoning, salt, and pepper. Mix well until fully combined.
4. Stuff the Shells: Using a spoon, carefully fill each cooked pasta shell with the cheese mixture. Place them in a baking dish lined with a bit of marinara sauce.
5. Layer the Sauce: Pour the remaining marinara sauce evenly over the stuffed shells. Make sure they are well-covered.
6. Top with Cheese: Sprinkle the additional mozzarella cheese over the top for that gooey, melted goodness.
7. Bake: Cover the baking dish with foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10 minutes, allowing the cheese to brown slightly.
8. Cool and Serve: Once done, let the stuffed shells cool for a few minutes before serving.

These steps guide you through the process of making your stuffed shells simple yet flavorful. Enjoy the delicious aroma that fills your kitchen as they bake to perfection!