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Rinse the cherries and remove the stems. Remove the pits, the most easy way is using a cherry pitter but a small knife or stick will work too. That will take a lot more time, but it is possible.
Fill a large pan with water and bring to a boil. Fill a second pan with water and sugar, use 300 gram sugar for 1 liter water. Create as much sugar-water as needed to fill the jars.
Heat the water until the sugar has dissolved completely.
Divide the pitted cherries over the jars and fill them with warm sugar-water. Put the lid on and put the jars into the pan with boiling water. If you don't like the noise of the jars touching the pan, you can put a kitchen towel on the bottom of the pan.
Depending on the amount of jars, you have to maybe cook them in batches or use a second pan. Cook the cherries for about half an hour on medium-high heat and then transfer the jars out of the water.
Put the jars upside down and let them cool off for half an hour. After that, theres no need to have them upside down while they cool off completely.
You can store the cherries for a long time, if you keep them in a dark spot without too many variations in temperature.