Put watermelon into a large saucepan or pot and stir in the pectin. Bring to a boil over high heat, constantly stirring when starting to boil. Stir in the sugar and lemon juice, continue boiling for 1 additional minute, constantly stirring. Remove from heat, skim off any noticeable foam and let cool 30 minutes. Ladle in prepared containers, cover and let cool in refrigerator.