Prepare Canning Equipment: Fill a large canning pot with water and bring it to a rolling boil to sterilize your jars for about 1 minute.

Make Watermelon Juice: Slice a half large watermelon, blend until smooth, and strain through a fine mesh sieve for about 2 1/2 cups of juice.

Combine Ingredients: In a large pot, mix watermelon juice, strawberries, lemon juice, and optional butter. Gradually add pectin while stirring.

Boil Mixture: Heat the mix over medium-high, stirring until it reaches a hard boil. Add sugar and boil vigorously for 1 minute and 10 seconds.

Canning Process: Ladle hot jam into sterilized jars, leaving 1/4 inch headspace. Wipe rims, seal, and process in boiling water for 10 minutes.

Seal and Cool: Remove jars and cool completely at room temperature. Listen for the pop of seals indicating preservation.