Not the original commenter, but it's really simple, you just need to mix fruit and sugar in a 1:1, 2:1 or 3:1 ratio and add a splash of water, and bring it to a low boil under constant stirring with a wooden spatula. Once all sugar is dissolved, take it off the heat and stir in some gelatine (powder or sheets - if you opt for sheets, soak them in cold water for 90 secs prior). Keep stirring until the gelatine is thoroughly dissolved as well, then put it into jars, pop on the lid while steaming hot, and chuck it in cold water for a minute to create a pressure seal. Then let it fully cool down at room temperature.

For plum jam particularly, I prefer a bit higher acidity so I'll go for a 3:1 fruit to sugar ratio, but that's your call, really. If you want to add some extra flavor notes, you can throw a handful of cloves in when cooking the mix (just get them out before filling it into the jars), and then after cooking, put a cinnamon stick into the jar right before you throw it into the water. That can remain inside until you first open the jar later.

I'd suggest to experiment around at first with smaller batches to get your preferred mix, and then go all in.