Put all the apple pieces through a centrifugal juicer. You will need 1000g of juice to make 1.5 kg (just over 3 and a quarter pounds) of jelly, to which you will add the juice of 1 lemon.
Skim off all the froth carefully, then leave the juice in the fridge for about 30 minutes to settle. You will see that a deposit has formed at top and another at the bottom.
Note: if you do not get the 1000g of apple juice, you will have to adapt your sugar and gelling weight, you can do it automatically with this little calculator: