Peel and pit the peaches, roughly chop and place in a large pot. Add the lemon juice and mash them together with a potato masher to desired consistency (I don't like my jam too chunky).

Stir in the sugar and bring to a boil over medium heat. Continue to boil, stirring frequently, until the jam reaches about 210-220 degrees F (99-105 C) and thickens, about 25-30 minutes (or don't bother measuring the temperature and just boil the jam until it looks good). You can test the thickness by dropping a little bit of jam onto a cold plate.

Ladle the jam into sterilized jars, wipe the rims clean, screw on the lids, and either cool and refrigerate/freeze or process in a boiling water bath for 10 minutes. Leave 1/4 inch headspace for canning, 1 inch for freezing.