1. Prepare the Watermelon Juice

• Cut the Watermelon: Begin by cutting the watermelon into chunks. Remove all seeds to avoid any bitterness or texture issues.
• Puree the Watermelon: Use a blender to puree the watermelon chunks until smooth.
• Strain the Puree: Strain the watermelon puree through a fine mesh sieve or cheesecloth to remove the pulp. You should be left with about 4 cups of smooth watermelon juice.

2. Cook the Jelly Mixture

• Combine the Ingredients: In a large, heavy-bottomed pot, add the watermelon juice, lemon juice, and powdered pectin. Stir well to ensure the pectin is fully dissolved.
• Bring to a Boil: Place the pot over medium-high heat and bring the mixture to a rolling boil. Stir frequently to prevent sticking and ensure even cooking.

3. Add the Sugar

• Stir in the Sugar: Once the mixture reaches a boil, add the sugar all at once, stirring constantly. Keep stirring until the sugar is completely dissolved.
• Boil Again: Return the mixture to a full, rolling boil and cook for an additional 1-2 minutes. Continue stirring to avoid burning and ensure a smooth texture.

4. Test for Doneness

• Check the Gel Consistency: To test if the jelly is ready, drop a small amount of the mixture onto a cold plate (you can place a plate in the freezer for a few minutes to chill). If the jelly forms a gel-like consistency and doesn’t run, it’s done. If the mixture remains runny, boil it for another minute and test again.

5. Jar the Jelly

• Remove from Heat: Once the jelly reaches the desired consistency, remove the pot from the heat.
• Skim the Foam: Using a spoon, skim off any foam that has formed on the surface of the jelly. This step ensures a smooth, clear jelly.
• Ladle into Jars: Carefully ladle the hot jelly into sterilized jars, leaving about 1/4 inch of headspace at the top. This space allows for expansion during storage.

6. Seal and Process the Jars (Optional)

• Wipe the Rims: Wipe the rims of the jars with a clean, damp cloth to remove any residue. This ensures a proper seal.
• Seal with Lids: Place the lids on the jars and screw on the bands until fingertip-tight.
• Process the Jars (Optional): If you want to store the jelly long-term, process the jars in a boiling water bath for 10 minutes. This step sterilizes the jars and helps create an airtight seal, allowing the jelly to be stored for up to a year.

7. Cool and Store

• Cool at Room Temperature: Allow the jars to cool at room temperature. You’ll hear a “pop” sound as the jars seal. Check the seals by pressing down on the lids; they shouldn’t flex up and down.
• Storage: Store the sealed jars in a cool, dark place for up to one year. If you skip the processing step, store the jelly in the refrigerator and consume it within a few weeks.