Prepare a very large pot or bowl with acidulated water in it (by adding lemon juice or ascorbic acid.)
Wash, peel, core, seed and slice the apples.
As you work, work in such a way that peeled apple never rests outside the water, to prevent discolouration.
Put drained apple slices in a large pot.
For every 1.25 kg (3 lbs / 20 to 30 medium apples / 12 cups prepped) in the pot, add about 125 ml (½ cup) water.
Bring to a boil, stirring occasionally, and then reduce to a good simmer for 5 minute to 20 to 30 minutes until all the apple is tender (how long will depend on the apple variety) and starting to break down.
Press through a food mill or sieve or put through a blender or food processor, OR skip this step if you want a chunky sauce.
Put sauce back in pot. If desired, sweeten it with up to 2 tablespoons sugar per litre / quart of sauce you estimate you have.
Bring back to a boil.
Pack hot applesauce into half-litre (US pint) jars or 1 litre (US quart) jars (OR SMALLER.)
If desired, add ½ tablespoon bottled lemon juice per half-litre/ US pint, one tablespoon to the litre / quart jars.
Leave 2 cm (½ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner or pressure canner.
Water bath / steam canner: half-litre (US pint) size jars for 15 minutes and litres / quarts for 20 minutes; increase time as needed for your altitude.
Pressure canner: half-litre (US pint) size jars for 8 minutes and litres / quarts for 10 minutes at 5 lbs (35 kPa) weighted gauge, 6 lbs (42 kpa) dial gauge (adjust pressure for your altitude when over 300 metres / 1000 feet)
(Note: if you do use smaller size jars, use processing directions for half-litre (pint) jars.