Cooking the jam and reading when it's done
This is the lesson where it actually turns into jam, so we'll go slow through it once and then you'll just do it a bunch of times and get a feel for it.
You've got your peaches prepped, measured, mashed or chopped the way you like them. You've got your pectin figured out, sugar measured, everything sitting next to the pot in the order you'll use it. If you skipped that lesson, go back, because jam moves fast once it starts and you do not want to be measuring sugar while the pot's already going.
Getting it going
Dump the fruit into the big pot first. If you're using pectin, this is where it goes in too, stirred into the fruit before any heat, unless your box says otherwise (some want it added later, read your box, I'm not going to pretend every brand is the same).
Turn the heat to high and stir it pretty much constantly. Not because it'll burn in ten seconds, but because it's easy to get distracted, and peach jam on the bottom of a pot is a bad smell you don't forget.
You're bringing it to what's called a full rolling boil. This is a real thing with a real look, not a gentle simmer. A rolling boil is still boiling even while you're stirring it. If you stir and it stops boiling, it's not there yet.
Once it hits that full rolling boil, if you're using pectin, you add the sugar all at once and bring it back up to that same hard boil. Set a timer for whatever your pectin box says, usually one minute. Boil it hard for that exact minute, stirring the whole time, then pull it off the heat.
The foam
You'll get foam on top. The little scummy foam, I call it, nothing fancier than that. Skim it off with a spoon as you go, or all at once at the end, doesn't matter much which. Some left in won't hurt you, it just looks a little cloudy on top of the jar. I skim mine because I like how it looks better, that's a preference thing, not a safety thing.
Doing it without added pectin
This is where "how do you know it's done" gets less exact, and I'll be straight with you about that. You're boiling the fruit and sugar together until it thickens on its own, and that runs anywhere from twenty minutes to what feels like forever depending on your peaches, your altitude, how juicy they were.
The test I actually use: put a small plate in the freezer before you start cooking. When you think the jam might be close, drop a small spoonful on the cold plate and stick it back in the freezer for a minute. Pull it out and push it with your finger. If it wrinkles and holds a line, you're done. If it just runs back together like syrup, keep boiling and test again in a few minutes.
The spoon test, where you lift a spoon out and watch how the jam falls off it, sheeting off in a clump instead of a thin stream, works too. But the cold plate is the one that actually tells me something. The spoon one I don't fully trust myself with.
I'll be straight, even doing everything the same way twice, I've had one batch set up firm and the next batch stay looser, same recipe, same peaches practically. I don't have a clean answer for why. I've made my peace with it. If it stays loose it still tastes like peaches, you just call it syrup and put it on pancakes and nobody complains.
Ladling it into jars
Once it's done, get it into your jars right away, don't let it sit and cool in the pot. Leave a quarter inch of space at the top, wipe the rim, every single time. One drip of jam sitting on that rim and your seal fails later, and you won't know why until you're pulling a soft jar off the shelf in October wondering what went wrong. It's the step everybody rushes, because by this point you're tired and there's jam everywhere, and it's the one that matters most.
I shot a ward canning night a couple years back, camera's basically glued to me at this point, and I got a picture of a grandma and a teenage girl standing at the stove together when a jar of jam sort of erupted a little, boiled over the pot edge, and they both just lost it laughing, this teenager who clearly did not want to be there at first and this grandma who'd done this her whole life, both cracking up over the same dumb mess. That's one of my favorite photos I've ever taken, and I've shot actual weddings. Canning makes people forget to be careful about how they look, which is the whole appeal to me.
One real caution here: steam off that pot is hotter than it looks and it's not the water that gets people, it's the steam rising up when you lift a lid or pull a jar. Keep your face and wrists back from it. I've got a scar that says I mean it.
Before next time
Get your jars lined up and your lids ready. Next lesson we're doing the actual water bath process from start to finish, timers and all.