Trim dark green and pink parts from watermelon rind; cut rind into small chunks (measure about 6 cups). Grind with food grinder, using coarse blade. Measure 4 cups pulp including juices. Set aside.
Squeeze juice from lemons. Reserve peel of two of the lemons. Measure 1/2 cup lemon juice; set aside.
Remove excess white from reserved lemon peels. Cut peel into very fine strips. Place lemon peel in 5-quart kettle. Cover with water; bring to boiling. Simmer 5 minutes. Drain, discarding cooking liquid.
Add the 4 cups ground watermelon rind and liquid to peel in kettle. Cover; simmer 10 minutes, stirring frequently. Drain liquid.
Measure and add water, if necessary, to make 2 cups. Return to kettle with rind and peel. Add lemon juice. Stir in pectin; bring to a full rolling boil.
Add sugar at once. Bring to a full boil. Boil hard 1 minute, stirring constantly. Remove from heat. Skim off foam. Pour into hot, sterilized half-pint jars or glasses. Seal.