Peel and core apples. I prefer to use an apple peeler that peels, cores, and slices the apples all in one step. However, a peeler and knife work just as well. Place peeled apples in a large bowl of water with a few tablespoons of lemon juice to prevent them from browning.
Put apples in a large stock kettle and add just enough water to prevent scorching. Stir often to check for apples sticking to the pan. Add small amount of water if needed.
When apples are soft and easily pierce with a fork, use an immersion blender to puree the applesauce. You can make it as smooth or chunky as you prefer.
Using a ladle and funnel, pour applesauce into prepared and sanitized jars. Run a knife down the inside edge of the jar to remove any visible air bubbles. Wipe the rim of the jar, top with a flat sealing lid and ring. Place in a rack in the water bath canner that has warm water in it.
When all the jars are filled and in the canner, fill the canner with hot tap water to one inch above the jars. Bring the water to a boil and then set the timer for 20 minutes.
While jars are processing, prepare a place for the jars to cool. It needs to be draft-free to guard against jars breaking from being hit by cold air. I use a double layer of dish towels on a protected area of my kitchen counter. (Please be careful if you place your hot jars on a wood table. The steam damaged the finish and my table had to be refinished.)
At the end of processing time, turn off stove burner. Use a jar lifter to remove jars from canner and set them on prepared, folded dish towels. Allow to cool overnight before checking the seal. You will know the jars are sealing when you hear the pop of the vacuum pulling the lid down.
When jars are cool, wipe with a damp rag to remove any sticky residue from the outside of the jar, label (I use a Sharpie to write the name of product and year canned on the lid), and store until ready to use.
FOR FRUIT FLAVORED APPLESAUCE: If you like applesauce that is flavored with other fruits, add the fruit at the beginning of the cooking process. I have added 2 cups of berries (blueberries, strawberries, or blackberries) or 3 bananas or 2 cups of chopped peaches.