I've been experimenting with pectin-free jam making this summer, gradually reducing the amount of sugar used each time. But I've been using different kinds of fruit as well, based on what was plentiful in the garden or store: blackberries, blueberries, strawberries and peaches. Each one has turned out differently, a little thicker, thinner, sweeter, tarter, but all much more flavorful than store bought, and they make nice hostess gifts as well.
Anyway, I started with washing the jars in hot, soapy water, and putting the glass in the oven at 250 to sterilize. Then, I zested and juiced two lemons (but a third wouldn't have been amiss, oh well!).
Carefully spoon in jars and seal, not too tightly. The hot jam in the hot jars will do the sealing work for you.
Homemade Peach Jam
Yields: Eight 1/2 Pint Jars
6 lbs white peaches, peeled and chopped into small chunks
2 lemons, zest and juice
2 cups sugar (more or less, to taste)
Combine ingredients and put into two large heavy pots, cook on medium high, stirring often to avoid burning, until the jam reaches 220 degrees or thick and syrupy (about 25 minutes), but before it gets dry. If necessary, mash the fruit for a finer texture. Carefully spoon into clean, hot jars and seal lightly. After the jars are cool you can tighten the rings, if necessary.