Prepare a very large pot or bowl with acidulated water in it (by adding lemon juice or ascorbic acid. This is to prevent the apple going brown.)
Wash, peel, core and slice the apples.
As you work, work in such a way that peeled apple never rests for long outside the water, to prevent discolouration.
Put drained apple slices in a large pot.
For every 2 kg (5 lbs / 16 cups ) of drained apple slices in the pot, add ½ litre (2 cups / 1 US pint) of water. Bring to a boil, then start timing and let it boil for 5 minutes.
Pack hot apple slices into half-litre (US pint) jars or 1 litre (US quart) jars.
Leave 3 cm (1 inch) headspace.
Top up with the blanching water or if there isn't enough, clean boiling water (such as from a kettle, for instance).
Leave 2 cm (½ inch) headspace after the filling liquid.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner or pressure canner.
Water bath / steam canner: either size jar for 20 minutes; increase time as needed for your altitude.
Pressure canner: either size jar for 8 minutes. 5 lbs (35 kPa) weighted gauge, 6 lbs (42 kpa) dial gauge (adjust pressure for your altitude when over 300 metres / 1000 feet)