Pour the watermelon in a blender and pulp it. Transfer the watermelon pulp into a saucepan and add the lemon juice. Mix 1/2 cup of sugar with pectin and add it to the mixture.
Heat the mixture and stir it constantly. When it just begins to boil add the remaining sugar. Reduce the heat slightly and continue stirring until it gets thick. Remove quickly the jam from the heat and let it cool. Put the jam into sterilized jars.