Remove the red flesh of the watermelon, leaving only one very thin layer of red flesh attached to the rind, about 2 mm/ 0.08 inches. Remove the green skin of the watermelon completely.
Chop the watermelon rind into even pieces of about 3-4 cm/ 1.2-1.6 inches. You should weigh the watermelon rind now and prepare more if necessary.
Place 1 liter/ 34 oz/ 4.2 cups water and the vinegar in a pot with a diameter of approximately 23 cm/ 9 inches and a height of 14 cm/ 5.5 inches (*See the marked paragraph above).
Bring to a boil, add the watermelon rind and let cook for about an hour or a bit more until the rind pieces are glossy and somehow transparent. It is a necessary step, the vinegar will help the watermelon rind pieces hold their shape and not get soggy.
Drain in a sieve, refresh with cold water and, when cool enough to handle, press with your hand to remove more of the excess water.
Weigh the watermelon rind again. You should have more or less 500 g/ 1.1 lbs cooked rind. Set aside.
Measure the sugar, you should use the same amount of sugar as you have of cooked watermelon rind, so about 500 g/ 1.1 lbs.
Place the sugar in the rinsed pot you used before. Add 250 ml/ 8.5 oz/ 1 cup water to the sugar and turn on the heat. Stir to dissolve the sugar. When the syrup comes to a boil, add the watermelon rind and the lemon juice.
Cook until a light syrup forms, it took me exactly 38 minutes, but as I have mentioned above, the cooking time depends on the size of the pot as well. I used a soup pot with a diameter of 23 cm/ 9 inches and a height of 14 cm/ 5.5 inches.
If your pot has a similar size the time should be OK, if your pot is wider the cooking time will probably be shorter, if your pot is narrower the cooking time will probably be a bit longer. The finished syrup should be light in color and have more or less the consistency of clear honey.
Place the jam into the sterilized jars and seal well. Keep in a dark cool place. If the jars are sterilized and well-sealed the jam should be good for a very long time.