Fill a boiling water canner half full of water. Bring this to a simmer. (At the same time, I start a teapot with water going, so I will have boiling water for the jar lids later on). Wash jars and lids in soapy hot water. Put lids in small bowl. Fill jars with water and put in canner on a rack, while it is simmering, to keep jars warm. (Or you can set jars on dish towel on a cookie sheet and keep in oven at 250 degrees for 20 minutes for same effect).
Put the finely chopped peaches (4 cups) into a large stock pot. Stir the lemon juice and the box of Pectin into the peaches. Bring mixture to a full rolling boil on high heat, stirring constantly (a rolling boil is when it is boiling hard, it won't stop bubbling even when stirred). Stir in all of the sugar quickly (all at once). Stir well to combine.
Bring the jam/pectin/sugar mixture back to a full, rolling boil. Once it is a full rolling boil, boil it for exactly 4 minutes, stirring constantly. When time is up, remove pan from heat, then skim off any accumulated foam (and discard foam). Ladle the hot jam mixture into hot, prepared jars. Fill the jars, but leave 1/8 inch headspace at the top of each jar. Insert a plastic knife into each jar a couple times to help remove air bubbles. Adjust the headspace if necessary. Use a damp cloth or paper towel to wipe down the rim and edges of the jar. You need it free of debris in order to get a good seal on the jar. Cover the jars with the hot, flat jar lid. Screw on the jar rings tightly. Lower each of the jars onto an elevated rack in the canner. The jars must be completely covered with water, and must have at least an inch of water over the top of the jar. Add more boiling water to canner, if necessary, to make sure.
Cover the canner; bring to a gentle boil. Once the water is boiling, process the jars for 10 minutes. When done, wait for a couple minutes, then carefully remove canner lid (lift lid away from you because of steam); carefully lift each jar out of water (with canning tongs), and place on dish towel on the counter to cool (do not put jars directly on counter cause you don't want temperature differences to possibly crack jars). You should hear "ping" sound as the jars seal properly.
After the jars cool off, you can check to see they sealed properly by pressing the middle of the lid. The jar should NOT spring back when touched. If it does spring back, then you will need to refrigerate that particular jar. Let the sealed jars stand at room temp for 24 hours, then store unopened in a cool, dark place for up to 1 year. If any jars do NOT seal, store in refrigerator and use immediately.
Peach Jam https://www.thegratefulgirlcooks.com/peach-jam-how-to-can-it/