Prepare 6-7 half-pint Mason jars and chill 2 small ceramic plates for testing.

Blanch peaches in boiling water for 1 minute, then ice water for 1-2 minutes. Peel, pit, and chop into 1/2-inch pieces (approx. 8 cups).

Mash the chopped peaches to your desired consistency, aiming for about 5 cups.

In a large pot, combine pectin, lemon juice, and 1/4 cup of the sugar with the mashed peaches. Bring to a full boil while stirring constantly.

Add the remaining sugar and return the mixture to a full rolling boil for 1 minute.

Test the jam's thickness on a cold plate. If it's too runny, add another 1/4 box of pectin and boil for 1 minute more.

Skim off any foam that forms on the surface. Let the jam sit for 10 minutes, then stir to redistribute the fruit.

Fill the prepared jars, leaving 1/4 inch of headspace. Wipe the rims clean and apply lids and bands, tightening them only partially.

Process the jars in a water bath canner, ensuring they are covered by 1 inch of water. Processing times vary based on altitude: 5 minutes at sea level, 10 minutes above 1,000 feet, and 15 minutes above 6,000 feet.

Cool the jars undisturbed for several hours or overnight. Once cooled, check that the lids have sealed properly and remove the bands.