Wash, cut and dice (and remove seeds if using a seeded melon) melon into 1-inch chunks. Place diced watermelon in a blender or food processor.
Cover and blend or process until smooth.
It should reduce to approximately 2 cups of watermelon puree. That (2 cups after the blender) is the amount you will need for the recipe. Pour it into a large pot. Bring watermelon juice to a slow boil.
Add the 3 Tablespoons of lemon juice to the pot.
In a small bowl, mix the dry pectin with about 1/4 cup of sugar (or other sweetener). Keep this separate from the rest of the sugar. If you are not using sugar, you'll just have to stir more vigorously to prevent the pectin from clumping.
Stir the pectin into the watermelon juice and put the mix in a big pot on the stove over medium to high heat (stir often enough to prevent burning).
It should take about 5 to 10 minutes to get it to a full boil (the kind that can not be stirred away).
When the berry-pectin mix has reached a full boil, add the rest of the sugar (or other sweetener). I recommend using some real sugar or honey, as jellies made with no added sugar, or just Splenda turn our rather flat tasting and runny.
After adding the sugar or sugar/sweetener blend, then bring it back to a boil and boil hard for 1 minute. Depending upon which type of jam you're making (sugar, no-sugar, Splenda, mix of sugar and Splenda or fruit juice) you will need to use a different amount of sugar and type of pectin.
I keep a metal tablespoon sitting in a glass of ice water, then take a half spoonful of the mix and let it cool to room temperature on the spoon. If it thickens up to the consistency I like, then I know the jam is ready. If not, I mix in a little more pectin (about 1/4 to 1/2 of another package) and bring it to a boil again for 1 minute.
IF YOU DO NOT OWN A CANNER/PROCESSOR -- While melon is coming to a boil, place 4-5 jars in a shallow baking pan and pour water to the half way point of pan. Place over electric eye (large) and bring to a boil. Place caps in small saucepan and cover with water and bring to a boil.
Fill jars to within ¼-inch of the top, wipe any spilled jam off the top, seat the lid and tighten the ring around them. Continue to boil in pan for 10 minutes.
Lift the jars out of the water and let them cool without touching or bumping them in a draft-free place (usually takes overnight) You can then remove the rings if you like. Once the jars are cool, you can check that they are sealed verifying that the lid has been sucked down. Just press in the center, gently, with your finger. If it pops up and down (often making a popping sound), it is not sealed. If you put the jar in the refrigerator right away, you can still use it. Some people replace the lid and reprocess the jar, then that's a bit iffy. If you heat the contents back up, re-jar them (with a new lid) and the full time in the canner, it's usually ok.
Once cooled, they're ready to store. I find they last up to 12 months. But after about 6 to 8 months, they get darker in color and start to get runny. They still are safe to eat, but the flavor and texture aren't as good. So eat them in the first 6 months after you prepare them!