We love making mannicotti, although a bit time consuming. I boil the noodles, then immediately put them in cold water. My filling is half shredded mozzarella cheese and half ricotta, with some parsley and garlic or other seasonings as you prefer. I like it too with chopped spinach also.
The tricky part can be stuffing the slippery shells. I have learned, if I get the cheese mixture really cold, I roll it, like playdoh, into a long line that will fit right into the noodle easily. Otherwise, if you go cramming the mixture in there, it can tear the noodles. I put some spaghetti sauce on the bottom of a glass baking dish - so the noodles dont stick - and then place the filled shells about 1/4 inch apart, then cover with remaining sauce, and more mozarrella, parsley, italian seasoning etc. Bake at 350 for 20-30 minutes to heat through.