This is what I do (and I'm a rookie, folks):
Adapted from here
For every 2kg of plums, I add approximately...
1 cup water
1.5kg white sugar
a squirt of lemon juice
(That's it! No pectin, no nothin'!)
- Boil the plums in a very large pot, whole, until their skins come off, the water is purple, and the the flesh is soft and taking on the same purple colour.
- (Meanwhile, you could be washing about 8 jars in hot soapy water, and setting up their lids to boil alongside your plums. You probably will only need 6 or 7 medium-sized jars, but better to have extra.)
- Pour off the water (and reserve it for smoothies and other creations) but reserve the cup of water needed for every 2kg of plums. (Both of these I estimate.)
- Empty the boiled plums into some other large container (or containers), and place a colander or steamer (I use a steamer because it has a flat base) over the large pot you used for boiling the plums. Slowly feed the boiled plums into the holey device of your choice, mashing the plums pulp through with a potato masher.
- Discard the stones (and any skin chunks).
- Add plum water, set aside earlier, along with a squirt of lemon juice.
- (Now you could place the washed jars into your cold oven, spacing them so they do not touch. Turn the oven on to 160 degrees C.)
- Return pulp to the stove and boil for 5-10 minutes. (When the oven reaches temperature, turn it off, and leave the jars waiting in the warm oven for filling.)
- Pour the the 1.5kg bag of sugar into a large glass bowl and microwave for 4-5 minutes on high (it should be warm to the touch...and careful, the bowl will be hot!).
- Add the sugar to the hot pulp, stirring until well dissolved.
- Put a small plate in your freezer.
- Heat on med-high (boiling) for 20 minutes, (stirring often), or until the jam reaches setting point -- test by dropping some jam onto the plate from the freezer and tipping the plate. If it gives a little, like jelly, but doesn't run, you're done! If it trickles across the plate, lick it clean (muaha!) put the plate back in the freezer and cook for 5 more minutes before performing the same test.
- Pour ladels-full through a funnel into your jars (warm from the oven) and close up, label, and finish off as you will.