'Can I make jam out of that?' is a something we should always be asking ourselves.
Always use a wooden spoon
This is complete bollocks. I use crappy metal spoons and I've made jam out of pretty much everything that ever lived and grew. So keep looking askance at that janky wooden thing hanging from your utensil rack or lurking in the second drawer. You don't need it. Don't go out and buy one.
Shouldn't I core that? For traditional jams, yes, probably. For jellies, NO. Never core anything from the rose family (apples, quinces, crabs, pears etc) if you're jellying them. The pectin lives in the woody parts including skin, pips, stems and strings and this is the evil that will make your jam strong. Trust me. If it looks like it belongs on the compost heap, throw it in the pot.
Shouldn't jam be like, super hard and stiff? I don't think so, but then I'm a sadist, not a masochist. I don't like to carve a piece out of the jar and this situation is more often than not the product of overexuberance with the jam setting sugar or pectin powder. If you can bounce it off the floor you've gone too far. The exception is quince, crabapple jelly etc; they should be the consistency of a good panna cotta, maybe 10 percent on each side of that golden mean. More about that later but for most jams, just past 'stiff sauce' is a good place to be, texture wise; it's not failure and will make no difference to its ability to keep, contrary to popular belief.
Apricot kernels are full of cyanide oh my gerrrrd. While cyanogenic glycosides sound bad, and you don't wan't to drink gallons of them, you will never be able to get enough into your jam to poison your annoying neighbour- quel dommage. You actually need to put the kernels from these fruits into the pot because of their pectin content. Don't panic.
It goes everywhere & I'm too lazy to scrub it off the jars! Me too. Which is why the single must have piece of equipment for any cackling jam-hag is the stainless steel jam funnel. I love my jam funnel; it is the Michael Fassbender of kitchen implements and I would lick that man off a dirty footpath which gives you an idea of the passion I feel toward the funnel. Buy one now. Make sure it fits the sort of jars you use. Put it in the oven with your jars to sterilize it. It has a thousand uses.
But it says you need citric acid! No, you don't. Its called a lemon and they are the jam hag's red right hand. Acidulate, and your jam will last a year in the dark. Lemon also supports the flavour of many fruits such as gooseberries, apricots and strawberries which lose their tang to the sugar. Industrial citric acid + tonnes of sugar + frozen pureed fruit is why commercial jam tastes like arse.
It's still sloppy :-( Never let jam outsmart you. Wait a few days. Plums and apples are particularly sneaky and like to pretend they're not going to set, but if you come back at the end of the week there's a sixty-seventy percent chance they will have firmed up nicely. If not, just pour it all into a pot and boil the everliving shit out of it for another 10 - 15 mins, re-sterilizing the jars. It will reduce in volume, but that was probably your problem in the first place- too much water added to the fruit when you're softening it. I still do it now and then. If you must, add some shop pectin powder. I've only ever resorted to extra pectin once and it does give you a hard, slightly grainy sort of finish if you're unlucky, but can salvage an expensive/difficult batch.
That cellophane business just seems whack to me. I only resort to cellophane if I run out of lids, and tend to use those jars first, but to be honest, Ive never had a cellophane + rubber band failure. Don't panic. Do read the sealing instructions, though.
Wild plums- bleurggg. Yes, they're pretty foul right off the tree. But the more sour the plum, the more glorious it will be in the jar. Floral, sharp and complex. Find a tree, watch it like a hawk and if you see kids ripping them off the branches and throwing them around, get out there with a big stick.
I don't have a recipe! When in doubt, it's the same weight of sugar to fruit, + the juice of two medium lemons. I can't really think of a drupe that this doesn't apply to. I tend to knock back the sugar to almost perilous levels, but this can be risky- sugar is a preservative that inhibits the growth of nasty microbes and you can cook yourself up a botulism special if you're not careful. The less sugar you use, the more quickly you should eat it. If you really want to scale back the sweetness, keep it in the fridge and eat it within a month or two.
Elderberries, wtf? I make gloopy inky syrup out of them which is just a non-setting jam, great for cakes, gravies, ice-cream, everything. Don't put the stems in- they're poisonous, and don't eat the raw fruit- it tastes like boot polish and I think it's also poisonous. Shake the insects off the clusters onto some paper before you start plucking the bunches or you'll find some yucky surprises in your jar. A few unripe ones won't make a difference. What does it taste like? Musky, sweet, medicinal, vaguely grapey, sort of like one of those weird liqueurs that lurked at the back of your parents liquor cabinet.
Medlars- WTF??? Yes, they are weird and creepy and look like a dog's bum but that is part of their charm. I bletted mine in a cardboard box in the bookcase and they were fudgy and delicious and made superlative jam. I recommend them if you can get hold of some; same procedure as quince. I think this was my pectin waterloo but that was because they were all too ripe; if you can secure a few greenish/unbletted ones to throw in the pot you should be fine. Chuck in a few apples with them if you can't. They taste caramelly and autumnal.
Good mixtures and jams you may not have considered gooseberry + up to half blackcurrant (you get a nice pink jam); black and redcurrant, whatever proportions, just chuck them in together; raspberry and whatever other berry is cheap at the time; nectarine (ambrosial), crystalized ginger and orange (not marmalade); pear- chop it up small, leave the cores whole and fish them out at the end, add fresh or crystalized ginger mmm!