Step 1: Sterilize your canning jars, lids, and bands. Wash them in hot, soapy water, rinse thoroughly, then either run them through a dishwasher on a hot cycle or submerge them in boiling water for at least 10 minutes. Keep jars hot until ready to fill. Simmer new lids in hot (not boiling) water to soften the sealing compound.
Step 2: Prepare the peaches. To easily peel, blanch peaches by dropping them into boiling water for 30-60 seconds, then immediately transfer them to an ice bath. The skins should slip right off. Pit the peaches and chop them into small pieces (about 1/2-inch dice) or mash them slightly, depending on your preferred jam texture. You should have approximately 6 cups of chopped peaches.
Step 3: In a large, heavy-bottomed pot or Dutch oven, combine the chopped peaches, granulated sugar, and fresh lemon juice. Stir well to combine.
Step 4: Bring the mixture to a full rolling boil over medium-high heat, stirring frequently to prevent sticking. Continue to boil vigorously, stirring often, for 15-25 minutes, or until the jam reaches its setting point. You can test for doneness by using a candy thermometer (220°F / 104°C at sea level) or by performing the cold plate test (spoon a small amount of hot jam onto a chilled plate, wait 30 seconds, then push it with your finger; if it wrinkles, it's ready).
Step 5: Carefully ladle the hot jam into the hot, sterilized jars, leaving 1/4 inch of headspace at the top. Wipe the rims clean with a damp cloth to ensure a proper seal.
Step 6: Place the hot lids on the jars and screw on the bands finger-tight. Do not overtighten.
Step 7: Using a jar lifter, carefully place the filled jars into a boiling water canner, ensuring they are fully submerged in water by at least one inch. Bring the water to a rolling boil and process half-pint or pint jars for 10 minutes (adjusting for altitude: add 1 minute for every 1,000 feet above sea level).
Step 8: Once processing is complete, turn off the heat and carefully remove the canner lid. Let the jars sit in the hot water for 5 minutes before carefully removing them and placing them on a wire rack to cool completely, undisturbed, for 12-24 hours.
Step 9: After cooling, check all seals. The center of the lid should be concave and not flex when pressed. Any unsealed jars should be refrigerated and consumed within a few weeks. Label your sealed jars with the date and store them in a cool, dark place for up to 1 year.