Wash the watermelon and cut it into wedges. Remove the rind, leaving some red flesh.
Using a vegetable peeler, remove the green skin from the rind.
Cut into small pieces.
Pour boiling water over the rinds. Bring to a boil and cook for about 10 minutes.
Drain the water and let the rinds cool.
Place the sugar in a small saucepan, add 1/2 liter of water and, while stirring, bring to a boil.
Pour this syrup over the crusts and cook over low heat for 20 to 25 minutes. Remove from heat and let cool for 2 to 3 hours.
Return to the heat and cook for 20 to 25 minutes. Let cool.
Return it to the heat a third time and cook for 20 to 25 minutes. Let it cool.
Check that the jam is ready. The pieces of watermelon rind should become almost transparent, a reddish-amber in color. A drop of jam should retain its shape and not spill onto the plate. If necessary, reboil the jam until it reaches the desired consistency.