Trim the pink flesh and the green outer skin from the rind. Cut into small strips, about 1" x 2". Cover with brine made by combining 3 quarts water and ¾ cup salt. Refrigerate for five hours or overnight.
Drain; rinse. Cover the watermelon with water and bring to a boil; continue cooking until fork-tender, about another 15 minutes. (Overcooking will cause the rinds to become rubbery.) Drain.
Combine sugar, vinegar, water and spice. Boil 5 minutes and then pour over watermelon; add lemon slices. Refrigerate overnight.
Heat watermelon in syrup to boiling; reduce heat to medium-high and for one hour. Pack the hot watermelon pickles loosely into clean, hot pint jars.Cover with boiling syrup, leaving ½ inch headspace. Remove air bubbles and adjust headspace if needed. Wipe rims of jars with a dampened clean paper towel; apply two-piece metal lids.
To seal: Submerge the full jars in boiling water (enough water so the jars are 1-2" below the surface); boil for 15 minutes (or slightly longer at higher altitudes). Remove jars and let them sit undisturbed at room temperature for 24 hours. Check seals.
