This is a very simple meal that I make in bulk and freeze for later use.
I start by cooking the shells according to the package instructions that are for baking afterwards. This is less time than al dente usually.
While that's cooking I prep the spinach. The frozen spinach was defrosted for several days in the fridge if possible. I then drained and squeezed it out. The least amount of water in them as possible! Husbands are good for this. It may be chopped already, but run a knife through it anyway. It seems unnecessary, but it does make a difference in the end product to me.
This was my second batch as I made a huge tray for family visits, but the same stuff goes in every time. Mix one egg beaten, 8 oz shredded Italian cheese mix (or create your own), 16 oz ricotta cheese, and 12+oz of Spinach (depending how much you like spinach). We typically use a 16 oz bag.
Once it's all mixed I toss in a few tablespoons of sauce then mix again until incorporated. My Grandma told me to do this though I'm not sure if it makes any difference. I will keep doing it despite my own thoughts.
I place a thin layer of sauce on the bottom of my cooking vessel. Usually Pyrex or Corningware, but tin trays work best for mass meals like this.
To fill the shells I use a mounded tablespoon or more as needed into each shell. Start with the whole ones as you will likely not use them all. Why try to fashion together a broken one when you may not have to.
Place the filled shells you intend to eat soon in the sauce lined dish. At this point you can place a small amount of sauce onto the top of each shell. It will keep it from drying out. Cover and place into fridge up to 24 hours.
When you are ready to cook place tin foil loosely over the top keep them from burning. I place it in while preheating as it makes no difference. Bake at 375 for about 40 minutes or until it has reached a safe temperature.
To store them, place filled shells onto a lined baking sheet into the fridge overnight. The next day place frozen shells into food saver bags (dry, soft setting or it could puncture the bag), or zip lock freezer bags. If you freeze them, defrost in the fridge overnight prior to cooking. Bake at 350 for about an hour with sauce like above.
You can even place them in small baking trays prepped with sauce and everything, however they do not last longer than a week or two using this method. They can lose their texture if left much longer. I only use this method if we have company and I don't have time to prep a few days before.