Using a sharp paring knife, remove green outer rind from watermelon rinds. Cut rind into 1-inch squares.

Make a cold water brine by mixing together the salt and 3 quarts of cold water, stirring to completely dissolve the salt.

Cover the watermelon with the cold brine and ice cubes. Stir and let sit for 3-4 hours.

Drain and rinse the watermelon rind under cold water in a colander.

Add the watermelon rind back to the sauceoan, cover with cold water, and bring the water to a boil for 10 minutes or until fork tender. Drain. Add the watermelon back to the saucepan or a large heat-proof bowl.

Pour the syrup/spice sachet over the watermelon rind. Stir in sliced lemon. Let the watermelon rinds and syrup stand overnight in the fridge.

The next day, heat the watermelon, spice sachet, syrup, and lemon slices in a large saucepan until boiling. Lower the heat and slowly cook the rind for 1 hour.

Pack pickled watermelon rinds in prepared pint jars. Ladle over hot syrup, leaving 1/2-inch headspace. Remove air bubbles with a wooden chopstick and adjust syrup as needed to fulfil headspace requirements.

Wipe each jar and jar rim with a clean kitchen towel before two-piece metal canning jars until fingertip tight.

Load jars into hot water canner and process for 10 minutes**, covering jars with at least 1-inch of water.

Remove jars from hot water canner with tongs, transferring them to a clean kitchen towel-lined space, leaving an inch of space between each jar. Let the contents of the jars come to room temperature (between 12-24 hours).

Check the seal of each metal jar lid, transferring any unsealed jars to the fridge. Jars with sealed lids can be stored in a cool, dry place away from direct light for up to 18 months.