@DobbleDobble plum jam is fantastic! Even commercial versions taste good.
Generic jam/jelly recipe: soften fruit, eg by cooking in as little water as possible. There are recipes for raspberry jam where the softening is done in the freezer. If you want to make a clear jelly rather than a jam, extract the juice (hang everything in a muslin bag and let it drip out; put it in a strainer with weights on, use a fruit press).
For every pint of fruit/juice add 1lb sugar. Bring to the boil in a vastly oversized pan, then keep it at a “rolling boil” (it’ll take for ever otherwise). Remove any unappetising scum that forms. First the excess water boils off, then the remaining sugar/fruit mix starts rising in temperature. Keep testing when it’s ready - drop a drip on to a cold plate, blow on it, hold it so the light shines across the surface, and push it gently with the tip of a finger - if you see tiny wrinkles on the surface, it’s done. Take the pan off the heat straight away.
What that hasn’t covered is how to get the plumstones out. We used to make damson jam when we lived in the SE, but that was 40 years ago and I can’t remember.
Pot the jam when it’s still hot and runny, the heat will sterilise any germs still remaining in the jar, and as it cools air pressure difference will seal the cap into position. Home made jam doesn’t need to be kept in the fridge even after opening.
Label the jar with fruit name and batch no - that way, if you find one jar didn’t set or went a bit toffee-like, you know which other jars will have that problem.
Finally, a word about pectin, which is what makes the jam set. Some fruits, eg quince, are naturally high in pectin, others are low, eg medlar, and need it adding, either via a high pectin fruit added in, or I believe you can buy it. Plums are high in pectin