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.
Combine watermelon rind, onion, peppers, and salt in a large bowl, stirring well. Chill overnight. 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. Ladle hot relish into sterilized jars, leaving 1/4-in. head space. Wipe rims with a damp cloth (to assure a good seal), screw sterilized lids on, and process for 10 minutes in a boiling water bath. (Click through for a complete tutorial on water bath canning if you've never done it before.)