Prep the peaches (peel and pit). Slice no more than 1 cm (½ inch) thick. Set aside in a large bowl or pot of water with some lemon juice or ascorbic acid in it to prevent discolouring.
In a large pot, mix together the Clearjel, cinnamon, sugar, almond extract if using, and water. Set aside.
Boil a large pot of water (4 litres / quarts). Add about 500 g / 4 cups / 1 lb of peach slices at a time to the boiling water. When the water returns to the boil after adding the peaches, time 1 minute, then remove the peaches with a slotted spoon or strainer, and put in a bowl or pot and cover to keep warm.
Repeat for all peach slices.
Put the Clearjel mixture pot on to cook over medium high-heat.
When it starts to thicken, which will be very soon, add the lemon juice.
Cook for 1 minute.
Add drained peach slices, then cook for 3 more minutes, then remove from heat.
Ladle pie filling into half-litre (1 US pint) or 1 litre (1 US quart) jars.
Leave 3 cm (1 ¼ inch) headspace (some experts advise even more headspace, such as 3.5 cm / 1.5 inches, is even better.)
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar for 30 minutes; increase time as needed for your altitude. See chart below.