In a food processor, blender, Ninja or bullet puree watermelon in batches.
Transfer watermelon puree to a stainless steel saucepan over medium-high heat. DO NOT BOIL. Add lemon juice and sugar and stir to combine. Using a thermometer heat to 190 degrees Fahrenheit, stirring occasionally. Because this recipe has sugar it will get to temperature rather quickly so keep your eye on the thermometer. Remove from heat.
Ladle mixture into jars leaving 1/4 inch headspace. Wipe rim, add hot lids and tighten rings just finger tight. Process in water bath canner at a boil for 15 minutes.
To reconstitute, mix one part concentrate with one part water, tonic water or ginger ale. Adjust concentrate to suit your taste.