1) 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).
2) 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 (I didn't bother measuring the temperature and just boiled the jam until it looked good). You can test the thickness by dropping a little bit of jam onto a cold plate.
3) Ladle the jam into sterilized jars and either cool and refrigerate/freeze or process in a boiling water bath for 10 minutes.