Over medium heat, place all of the ingredients (except for the butter) into a large saucepan, including: 6-7 cups of pitted cherries (frozen or fresh), sugar, cornstarch, salt and water.
Stir to dissolve the sugar. More water will created in the pan as the cherries start to cook. I typically use my wooden spoon to do all of the mixing for fruit pies. Bring the mixture to a boil. Once you see small bubbles forming around the edge of the pan, it's time to continue stirring and stay with it as it thickens. If you walk away for the last 5-10 minutes, you could burn the bottom of the filling and your pot. From start to finish, it will take a total of 30 minutes to cook the cherries down to a nice thickness, perfect for pie filling.
Once the filling is done cooking, pour the hot prepared filling into a storage container to cool or pour directly into a prepared pie crust.
If you are storing it for later use, cover it completely and store in the refrigerator for up to a week. Store in the freezer for up to a month.