A shortcut to start you off: If you have a food processor you can just toss roughly chopped watermelon rind, onions, and peppers into the bowl of the processor and pulse until they're a good size. I like mine in about 1/8 - 1/4" pieces. If you don't have a food processor, just chop them finely by hand.
4 cups chopped watermelon rind, 2 cups onion
Combine watermelon rind, onion, peppers, and salt in a large bowl, stirring well. Chill overnight.
1 red bell pepper, 1 jalapeno pepper, 1 1/2 teaspoons sea salt
Place in a colander to drain; rinse thoroughly and drain again.
Transfer to a large stock pot and add remaining ingredients; bring just to a boil. Simmer for 10 minutes. At this point, the relish is done. You can just put it in jars and then in the refrigerator to use right away or give as gifts.
If you'd like your relish to be shelf stable, you'll need to process it.
Prepare for Canning
Wash the jars you'll use, making sure each is clean and free of nicks in the rim, which could impede sealing.
Wash the lids and rings in hot soapy water. (If you're using non-Ball brand lids, prepare as suggested by manufacturer.)
Place empty jars in a canning pot or large stock pot with enough water to cover by an inch or two, cover pot, and set on high heat to boil. It can take awhile for the water to heat, so get it started before you begin making the recipe.
Canning the relish:
Ladle hot relish into pint or half-pint jars, leaving 1/2" head space. A canning funnel makes this easy.
Wipe jar rims to remove any relish that may have spilled. A clean rim is essential to a good seal.
Set jar lids in place. Screw bands on finger tight.
Use a jar lifter to gently submerge jars into boiling water in canning pot. Water should cover the top of the jars by an inch. The water will cool somewhat in reaction to the addition of the jars. Return the water to a simmer and set the timer.
Process for 15 minutes 0-6,000 feet altitude; 20 minutes over 6,000 feet.
Allow jars to cool overnight.
Check for seal: the lids should feel solid and slightly indented. If they flex, that are not shelf stable and should be refrigerated and used first.
Wash jars, remove rings, and store in a cool, dry place for a year.
Notes
I used yellow onions for this recipe, but you can use any kind of bulb onion you have on hand.
This recipe, as made, tests at 3.5 pH, making it safe for water bath canning. Altering ingredients will impact the pH of the final product making it potentially unsafe for water bath canning.
Boiling lids or heating above 180°F as once recommended can damage the sealing compound.