Wash and dry the cherries well, remove the stone and the pith, then pour them into a container large enough to prevent them from clumping together;
Pour the sugar over the cherries, cover the container with a clean cotton cloth and leave to rest overnight;
The next day, drain the cherries from the juice they will have released and transfer them to a container, pour the juice into a heavy-bottomed pan and cook over a moderate heat until a light foam appears on the surface;
As soon as it appears, remove the pan from the heat and pour the juice over the cherries;
Cover with a cloth and let it rest another night;
Repeat the operation the next day;
Pour into sterilized jars, making sure that the syrup completely covers the cherries, close hermetically, turn the jars upside down and leave them to cool;