Crush watermelon with a potato masher inside a large stainless steel saucepan. Cover and heat gently over medium-low heat for approximately 5 minutes. Remove from heat and crush thoroughly.
Transfer to a strainer lined with several layers of dampened cheesecloth set over a deep bowl. Allow this to set undisturbed for 2 hours.
Measure 2 Cups of watermelon juice. If you do not have enough juice, crush more watermelon or (See Note Below!)
Prepare your canner or large deep pot, jars, and lids.
Transfer your watermelon juice to a clean, large, deep saucepan
Add in your vinegar, lemon juice, sugar, and lemongrass
Over High heat, stirring constantly, bring to a full rolling boil that cannot be stirred down.
Stir in Pectin and boil hard, stirring constantly for 1 full minute.
Remove from Heat and quickly skim off foam.
Quickly pour your hot jelly into your hot jars, leaving 1/4" headspace. De-Bubble.
Clean the rims of your jars with vinegar and center your lids. Add your bands, and only tighten them until you feel resistance. (See Note Below!)
Place your jars inside your canner or deep pot. Make sure that your jars are completely covered with water. (Approximately 1" above the lids)
Bring to a rolling boil and process for 10 minutes.
Remove lid and let sit for 5 minutes. Remove the jars and let sit for 12-24 hours or until fully cooled. Check your seals and store.