Finely dice the watermelon flesh, removing the seeds. Put into a pan, add 50 ml water and heat slowly, stirring occasionally. Simmer for 5 minutes, then set aside and leave to cool.
Stir the sugar into the cooled fruit, add the lemon juice and leave to stand for about 2 hours. Then reheat, stirring, and boil vigorously for 4-6 minutes. Pour into prepared jars, filling them to the brim, and seal well. Stand the jars upside for 5 minutes, then turn over and leave to go cold.
Label, giving content, type of sugar and date.