Pick a variety of peaches whose skin is easy to peel. Peel them, remove the pits and cut them into cubes. Pour the fruits into a deep tray and sprinkle with the sugar.
Leave it overnight (at least 12 hours). The next day put the fruits to bake in a moderate oven until the syrup reaches the typical consistency for jams - a drop of it must not run along the plate.
Take the peach jam out of the oven, stir with the citric acid and distribute into jars while still warm. Close quickly with caps and turn them upside down until they cool.