First, inspect all the apples. Choose the best, unblemished ones. If necessary, use a knife to cut out any rotten parts and rinse all the apples in clean water.
Now you need to pierce the apples with a sharp toothpick. I recommend piercing the fruit in as many places as possible, as this will release the juice faster and the jam will have a richer flavor.
Now we'll make the syrup. Place 2 kg of sugar in a large, deep-bottomed saucepan. Add water and bring to a boil. Keep the heat low so you can see the sugar melt and stir gradually until the mixture reaches a smooth consistency.
Add lemon or citric acid. A reaction will occur, and white foam will appear, but don't be alarmed. Continue stirring until the syrup becomes crystal clear and transparent as glass (about 6 minutes).
Add the pre-prepared apples into the syrup and cook for another 13 minutes over medium heat.
Remove the pan from the heat and cover it with a clean plate of a suitable diameter, then place the jar on top. This will create a weight to help the apples soak up the juice. Let the entire setup sit overnight or for 12 hours.
The next day, bring the apple syrup to a boil. Simmer over low heat for about 5 minutes. Then remove again and cover with a plate. Leave it like this for 12 hours.
We're approaching the finish line. Place it back on the stove and bring to a boil (all at low heat). Let it simmer for 10 minutes and observe the consistency. Ideally, a drop of jam shouldn't spread across the surface of a cold plate. Otherwise, continue to simmer.
Pour the jam into sterilized jars.