Prepare spaghetti noodles and brown the ground beef, as noted above.
Preheat oven to 350 degrees.
Set aside a couple of handfuls of shredded mozzarella cheese to cover top of the casserole. (You don't need it quite yet, obviously.)
Combine prepared spaghetti noodles, browned ground beef, spaghetti sauce, vegetables (if added), and the remaining mozzarella cheese (that you didn't set aside already).
I sometimes combine everything in the 13" x 9" pan that I'll bake it in, and sometimes I use a large mixing bowl. The large mixing bowl is easier, but sometimes I just don't want to dirty another dish.
Put your combined spaghetti mixture into a 13" x 9" cake pan.
Cover the top of the mixture with the shredded mozzarella cheese that you set aside.
Cover pan with aluminum foil. Bake for 45 minutes in a 350 degree oven.
Cool and enjoy!