I recommend that you fill uncooked manicotti noodles instead of cooking them first. Most recipes ask you to cook the noodles first, but IME, I find it virtually impossible to keep cooked noodles intact while you stuff them.
You can use normal Manicotti noodles, but add a little more cooking time, and if the original recipe seems dry, a bit of extra water.
My off-the-top-of-my-head recipe:
15 oz ricotta cheese
2 cups mozzarella, shredded
1/2 to 1 cup Parmesan
chopped frozen spinach, thawed (optional)
a box of manicotti noodles (uncooked)
a jar of your favorite spaghetti sauce, or the equivalent amount of homemade spaghetti sauce (slightly watery)
Mix the ricotta, a cup of mozzarella, and half of the Parmesan with the eggs and spinach. Put the mixture in a gallon-size zip lock bag, and cut a small hole in one corner of the bag. Squeeze the cheese mixture into the manicotti noodles until all the cheese mixture is gone and/or you don’t have any more room in the pan. Put the filled manicotti noodles in a lightly greased pan, pour the spaghetti sauce on top, and cover with foil. Bake at 350F for 40-45 minutes. Uncover the pan (by this time, the noodles should be completely cooked), sprinkle the rest of the mozzarella and Parmesan on top, and bake for another 15 minutes, until the cheese is melted and starting to brown.