Peel the watermelon, removing all the green skin and pink flesh, leaving only the firm white rind.
Cut the rind into bite-sized cubes.
Place the rind in a large pot, cover with water, and simmer for 20–30 minutes until just tender. Drain well.
In another large pot, combine the sugar, 1 litre of water, lemon juice, cinnamon sticks, cloves, and ginger (if using). Bring to a boil, stirring until the sugar dissolves.
Add the cooked watermelon rind to the syrup.
Simmer gently for 2–3 hours, stirring occasionally, until the syrup becomes thick and the rind turns translucent.
Remove the cinnamon sticks and cloves.
Spoon the hot preserve into sterilised jars and seal immediately.
Allow the jars to cool completely before storing.