First, wash/sterilize your jars and put them on a cookie sheet in the oven at 125 or 250, whatever makes you feel good. If you don't keep the jars hot - the boiling hot jelly may make your jars crack and break, causing a big unhappy mess. Boiling hot jelly will sizzle in the jars kept at 125 and you won't be able to grab the jars with your hands so have pot holders ready.
Next, gather your plums and wash them really well to get off all the bird poop and bugs. Or not - it's all going to get strained anyway and you might like the protein. Cut out any bad blemishes or parts you wouldn't want to eat. Your plums should look all clean and pretty:
I never add juice to the pot for the plums -- but I squish them hard with my hands as I put them into the pot -- I use both hands and squish a small handful of plums. Once the bottom of the pan is covered, I turn on the heat on low and continue adding smashed plums to the pot. This helps the juice release more quickly.
After you've smashed up all the plums and they've filled the pot with juicy goodness, turn them into a big colander or sieve lined with 2-4 layers of cheesecloth over a big bowl or pot to collect the juice.
You can stir the plums to release more juice, or pick up the corners of the cheesecloth from time to time to give a wiggle and shake. Once you've collected all your juice, you can store it and finish it another night or put it back in a pot and start thinking about flavor accents -- like habanero or jalapeno pepper or lavender.
In this picture, you can see the jars I used to store the juice in the refrigerator - you can also freeze the plum juice if you want to make it much later or really have a lot of plums.
Adding pepper can be tricky - so you have to start with a little, bring it to a simmer for five minutes and then taste it (let it cool, first!). Add more chopped pepper if you want more heat.
Simmer a bit and then strain out all the pepper (or lavender or other herbs) and put back in the pan.
Add the lemon, sugar and pectin as instructed by your pectin manufacturer. I use Pomona's pectin - so I put the lemon juice & calcium water into the pot, and mix the pectin in with the sugar really well using a wire whisk before adding. Then I follow the instructions for bringing back to temperature and testing for set.
Pour into hot jars and put the lids on and process in hot water bath. Then, set the jars out to cool and wait for the sweet plink of success (some people skip hot water bath with jelly -- this isn't recommended by the folks at Ball but it's done often since the simmering jelly is hotter than boiling water anyway).