You can fill a ziplock bag with the cheese filling, seal it and cut a small opening into the corner. Squeeze the cheese filling into the shells from the opening in the bag. Good luck.
I'll politely disagree with my friend bigpan on the squirt bottles---btw you can buy clean ones at either the supermarket or restaurant supply for about 69 cents. The neck is too narrow to let let something as thick as ricotta filling pass through.
You can use an old ketchup squirt bottle (like the kind in diners) and fill with the stuffing mix. Cut the tip of the ketchup bottle so you can easily squeeze and fill. Good luck.
Unless you have a pastry bag the size of an eye dropper you will find it impossible to stuff small shells. Use a good,tomato sauce (preferrably homemade)that you like, good quality ricotta, and your favorite herbs. But use large shells; conchiliglione.
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