Measure and dice the watermelon into small, uniform pieces, then transfer to a large saucepan. Use a sharp knife and a cutting board for clean, precise cuts.
Add the sugar and lemon juice to the watermelon in the saucepan. Stir well to coat all pieces evenly and let sit for 10 minutes to draw out moisture.
Place the saucepan over medium heat and bring the mixture to a gentle boil, stirring frequently with a wooden spoon until the sugar dissolves completely and the fruit starts to release juice.
Reduce the heat to low and simmer the mixture, stirring occasionally. Continue cooking until the jam thickens and the watermelon pieces become translucent, approximately 45–60 minutes. The jam should hold a slight mound when spooned and show a glossy appearance.
Once the jam has reached the desired consistency, remove the saucepan from heat. Ladle the hot jam into sterilized jars, leaving appropriate headspace. Seal and let cool completely before storing.
Label the jars and store in a cool, dark place or refrigerate. The jam will develop a richer flavor after a day or two.
