Place the apple peels and cores in a large pot.
Add water until the apple is just covered.
Boil until the apple cores are soft and mushy. The water level will have decreased as well.
Strain out the apple scraps.
Measure the liquid and pour it back into the large pot.
For each cup of lquid you add back, add 1/2 cup sugar and 1 TBSP lemon juice.
Bring to the boil and cook, stirring often until the mixture reaches gel point.
If you have a candy thermometer, it needs to read 105°C (220°F), though you will need to keep a close eye on the mixture when it starts getting around this temperature.
If you don’t have a thermometer, you can use the jam / cold plate test. Before starting your jam, simply place 3 or 4 small plates in the freezer. When you think the jam may be ready, place a small teaspoon amount of jam onto one of the cold plates from the freezer and let it sit for about 30 seconds. Then run your finger through the middle of the blob of jam. If the line starts to fill back over, it means the jam is too runny and is not ready yet, so keep cooking and repeat again in about five minutes. If the line remains, then the jam has reached setting point and is ready.
Ladle into sterilsed jars leaving 1/2 inch of headspace. Screw on the lids.