Cook pasta (any kind works: angel hair, spaghetti, rotini, etc) for a 1-2 minutes less than pkg directions. (ie if instructions say to cook for 8 minutes, cook for 6-7)
Warm marinara sauce in a saucepan. Add cooked ground beef to sauce, if using.
Place a small amount of sauce on bottom of whatever baking pan you're using. I use a 9x13 for five of us with leftovers. (see notes)
Layer in pasta, mozzarella cheese, pepperoni, and any other ingredients until the pan is full or the ingredients are gone. The last two layers should be sauce and mozzarella cheese.
Sprinkle the parmesan cheese on top.
Bake, uncovered, at 350° for 25-30 minutes until bubbly and cheese is slightly browned. (Can be cooked for less time since all of the ingredients are pre-cooked, but the baking process allows the flavors to meld.)
Let sit for a few minutes, so you don't burn your mouth eating it. ;)