Wash the cherries, cut them by half and remove the stones. Wrap the stones in cheesecloth and tie with kitchen twine.
Place the pitted cherries with the brown sugar, lemon slices and the cheesecloth wrapped stones in a heavy saucepan.
Start cooking the cherries over low heat, stir until the sugar dissolves into the cherry juice. When the juice is warm (40°C) stir in the caster sugar and pectin.
Once the jam reaches boiling point, let it simmer for 20 minutes. Stir frequently and remove impurities.
Transfer the hot jam into the sterilized jar and seal. Place the jar upside down and allow it to cool.
Notes
Less pectin will give a soft to runny jam texture and a high pectin content will make the jam more solid.