Blend watermelon pieces in blender for about 1 minute on medium speed.
In large kettle, mix melon, sugar, and lemon juice.
Bring to a boil.
Stir in liquid fruit pectin and boil fifteen minutes, stirring.
Skim off the foam and pour into jars.
Wipe rims, place lids, screw on bands finger-tight and process in a boiling water bath for ten minutes.
Remove to a protected surface to cool undisturbed.
Note: this is an adopted recipe, and needs a bit of tweaking. I plan on making the jelly with a thermometer in the future to ensure that the results are consistent.
Note #2: I've received questions about the 15 minute time listed in the recipe. Almost all of the watermelon jelly recipes I've found doing a search are for 1 minute. As this recipe has already been reviewed as is, I don't want to alter it too much. But as an alternative, I'd like to give you a recipe from Epicurious.com, whom I've found to be a reliable source. Watermelon Jelly, courtesy of Epicurious.com: 4 cups seeded, diced watermelon, 3 1/2 cups sugar, 2 Tbs lemon juice, 1/2 of a 6 ounce pkg of liquid fruit pectin (1 foil pouch). Instructions: Place diced watermelon in a blender container or food processor bowl. Cover and blend or process until smooth (should have 2 cups watermelon puree). In a 6 to 8 quart kettle combine the watermelon puree, sugar and lemon juice. Bring the mixture to a full rolling boil (a boil that cannot be stirred down) over high heat, stirring constantly with a long handled wooden spoon. Stir in the pectin all at once. Return mixture to a full rolling boil; boil hard for 1 minute, stirring constantly. Remove kettle from heat; skim off foam. Ladle jelly into clean, hot half-pint jars, leaving 1/4-inch head space. Wipe jar rims; adjust lids. Allow to cool completely away from drafts, then store in cool, dark place. Makes 4 half pints.
Note #3: from RecipeZaar user Foggy Kitchen come these helpful comments. "Mine did not gel and I had to research why not. Follow directions! Detail on the Pectin box distinctly separates fruit w/pectin -vs- bowl of sugar on side to be added LATER, then briefly fast boiled (the one minute reference).".