Prep canning supplies. Heat jars in the oven at 200℉, or by using the dishwasher or placing in hot water. They don't need to be sterilized, but need to be hot so they don't crack. Wash rings and lids in soapy water and place in a small saucepan of simmering water. Fill the canner ¾ full with water and place on high heat to bring to a boil. Keep everything hot until you're ready to fill them.
Prep the apples. Wash, peel, core, and slice the apples into uniform pieces.
Make the syrup. In a large saucepan, whisk together the granulated sugar, brown sugar, Clear Jel, ground cinnamon, ground nutmeg, and salt until well combined. Gradually stir in the water and apple juice (or cider) into the sugar and spice mixture. Add the lemon juice. Bring the mixture to a boil over medium heat, stirring constantly until it thickens.
Add apples. Gently add the prepared apple slices to the boiling syrup. Reduce the heat to medium-low and let the apples simmer for about 6-8 minutes until they become tender but not mushy. Stir occasionally to make sure everything cooks evenly. Add vanilla if using.
Fill and seal jars. Carefully ladle the hot apple pie filling into the prepared canning jars, leaving about ½-inch of headspace at the top. Use a non-metallic utensil to remove any air bubbles by gently tapping the sides of the jars. Wipe the jar rims clean to make sure you get a proper seal. Place the lids and bands on the jars, tightening the bands until they are just fingertip-tight.
Process. Process the filled jars in a boiling water canner for 25 minutes (adjust processing time for altitude if necessary). Make sure the jars are fully submerged in water during the entire processing time. If you are processing pints, they need 25 minutes as well.
Cool and store. Once the processing time is up, carefully remove the jars from the canner and place them on a clean kitchen towel. Let the jars cool completely. As they cool, you should hear a "pop" sound as the lids seal. Check that the lids are properly sealed before storing the jars in a cool, dark place. If any do not seal, place them in the fridge and use them within a few weeks.