You can pick your own, or buy them at an orchard or grocery store. Here in western Massachusetts, the UMass Cold Spring Orchard reasearch and education facility sells all kinds of apples. If you are buying your apples from a commercial orchard, ask for "seconds" or "drops". Apples with a bruise or odd shape make great sauce and cost less than perfect apples. Not all orchards sell "seconds", but they're a bargain when you can get them. A half bushel of apples will yield about 12 pints of sauce.
Get the jars ready before you start cooking the apples. The dishwasher is fine for the jars. Leave the jars in the dishwasher on "heated dry" until you are ready to use them. If you don't have a dishwasher, wash the jars in hot, soapy water and rinse, then dip the jars in boiling water. Keep the jars in hot water until they are used. Keeping them hot will prevent the jars from breaking when you fill them with the hot applesauce.
Put the lids into a pan of hot, but not boiling water (manufacturer's recommendation) for 5 minutes, and use a magnetic "lid lifter wand" or a slotted spoon to pull them out.
Wash the apples in plain cold water. Chopping them is much faster if you use an apple cutter - you just push it down on a peeled apple and it cuts it into segments. Apples get brown edges fast so as you cut them, drop them in a bowl of lemon water (1 whole lemon squeezed into a quart of water) until you have enough to begin cooking.
Cooking the apples is pretty simple. Put 1/4 cup of fresh squeezed lemon juice and 1/4 cup of store-bought apple juice into a 4 or 5 quart, thick-bottomed pot. Drop in the chopped apples until the pot is 3/4 filled. Put the lid on, and the heat on medium-high. When it starts to boil turn the burner down to medium and cook, stirring often, until your apples (and in my case a pear or two) are soft. Use a potato masher to squash the apples into a slightly chunky sauce and take a taste – if your sauce is a little too tart or too bland, you can add sugar one teaspoon at a time until you like the flavor. Go easy on the sugar, as a little goes a long way when it comes to applesauce. You can add cinnamon if you like. Again, start with a sprinkle because, like the sugar, a little goes a long way.
The applesauce does not need any further cooking; just keep it hot until you get enough made to fill the jars you will put into the canner Canners hold seven jars at once. Fill the canning pot 1/2 way with water and set to boil.