Wash the blueberries in cold water. The more you wash them, the more you avoid them falling apart.
Put the washed blueberries in a pot, add granulated sugar, and leave them overnight. The next day, the moment it starts to boil, add lemon juice immediately.
Pour the jam on a tray. Cover it with a thin cheesecloth and leave it in a place that will see the sun.
Stir the blueberries occasionally so as not to scatter them.
When the jam thickens, take it out of the sun, put it in a jar, and store it in a cool place.