Dice the watermelon flesh into small uniform cubes and measure 4 cups, then transfer to a large mixing bowl.
Add the sugar and lemon juice to the watermelon in the saucepan, stirring until the sugar is evenly distributed. Let sit for 10 minutes to allow the mixture to macerate and release juice.
Pour the mixture into a large saucepan and place over medium heat. Bring to a gentle simmer, stirring occasionally. You should see the mixture start to thicken and bubble gently after about 10-15 minutes, with a fragrant aroma developing.
Reduce heat to low and continue simmering for an additional 15-20 minutes, stirring frequently. The jam will thicken further, reaching a glossy, jelly-like consistency when it passes the 'cold plate test' (a small spoonful gels when cooled).
Remove the saucepan from heat and carefully ladle the hot jam into sterilized jars. Allow to cool at room temperature, then seal and refrigerate for up to two weeks. The jam will set further as it cools, with a smooth, translucent appearance and vibrant pink color.
