Pop the stones out of the cherries before you start your jam making session. While French purists would insist that the stones remain in place for the full-on flavour experience, I always like to think about my teeth and the cost of dentistry should I be unfortunate enough to munch into a stone stoked cherry and do some horribly expensive damage.
Save the cherry stones and crack them with a meat tenderising hammer or a rolling pin. Put the cracked stones into a little piece of muslin and tie it up securely with string.
Gently heat the cherries with the juice of the lemons, 4 tablespoons of water and the sugar until the latter dissolves completely.
Bring this to the boil for 5 minutes.
Remove from heat and stir in the liquid pectin to mix thoroughly.
Return the pan to the heat and reboil until you reach the setting point, see my jam making page for full explanation on setting point of jams etc.
Skim off any foam from the surface.
Allow to cool for a few minutes before carefully ladling into sterilised jars.