Place watermelon in blender or food processor and blend or process
until smooth. You should end up with 2 cups puree.
In large cooking pot combine watermelon puree, sugar and lemon juice.
Bring mixture to a full rolling boil. Add pectin all at once. Boil
hard for 1 minute, stirring constantly. Remove from heat; skim off
Ladle jelly into clean hot half-pint jars; seal. Process in a boiling
water bath for 5 minutes.